Starting from the same https://git.kernel.org/pub/scm/git/git.git/commit/?id=5476e1efded5
(see previous patch) git changed output when reporting error about a bad
pack. Before that patch it was something like

        $ git -c fetch.fsckObjects=true fetch-pack --thin --upload-pack=git -c uploadpack.allowAnySHA1InWant=true -c uploadpack.allowTipSHA1InWant=true -c uploadpack.allowReachableSHA1InWant=true upload-pack /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/3/incomplete-send-pack.git 46094318d7ea2dab446294556097361409ca1e84 </dev/null
        warning: no common commits
        remote: I: x-missing-blob/hook-pack-object is running ...
        remote: Enumerating objects: 4, done.
remote: Counting objects: 100% (4/4), done.
remote: Compressing objects: 100% (3/3), done.
        remote: Total 4 (delta 1), reused 0 (delta 0)
        fatal: object of unexpected type                               <-- NOTE
        fatal: unpack-objects failed                                   <-- NOTE

and after the patch it became

        $ git -c fetch.fsckObjects=true fetch-pack --thin --upload-pack=git -c uploadpack.allowAnySHA1InWant=true -c uploadpack.allowTipSHA1InWant=true -c uploadpack.allowReachableSHA1InWant=true upload-pack /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/3/incomplete-send-pack.git 46094318d7ea2dab446294556097361409ca1e84 </dev/null
        remote: I: x-missing-blob/hook-pack-object is running ...
        remote: Enumerating objects: 4, done.
remote: Counting objects: 100% (4/4), done.
remote: Compressing objects: 100% (3/3), done.
        remote: Total 4 (delta 1), reused 0 (delta 0), pack-reused 0 (from 0)
Receiving objects: 100% (4/4), 377 bytes | 377.00 KiB/s, done.
Resolving deltas: 100% (1/1), done.)
        fatal: did not receive expected object 737592d2855213009bd3fa689167fe9a8363e45f         <-- NOTE
        fatal: index-pack failed                                                                <-- NOTE

-> Adjust the test to detect the second case as also ok to avoid failing
tests with Git ≥ 2.31.

Pull test verifies that pulled tag objects become gone after becoming
represented via specially-crafted commits. The "gone" is verified by
removing all unreachable objects and asserting that looking up an object
by original tag sha1 results in error.

However there was a thinko in "removing all unreachable objects" - this
step was implemented via `git prune`, but as documented, `git prune`
removes only loose unreachable objects, while unreachable objects that
are already in packs remain intact. From `git prune --help`:

    Note that unreachable, packed objects will remain. If this is not desired, see git-repack(1).

So previously we were lucky that running the tests did not pull objects
from test repositories into packs and plain `git prune` was enough.
However starting from Git 2.31 fetching objects from test repositories
pulls some of them as packs and the test starts to fail because original
tag object remains alive:

    === RUN   TestPullRestore
    # creating root commit
    # building "already-have" index
    # git  b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/HEAD
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/config
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/description
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/info/exclude
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/refs/.keep
    # building "already-have" index
    # git  b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/HEAD
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/config
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/description
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/info/exclude
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/packed-refs
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/refs/tags/tag-to-blob
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/refs/test/ref-to-tree
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/world.txt
    # git  b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/HEAD
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/config
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/description
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/info/exclude
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/refs/.keep
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/file 2
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/nongit.git/latest
    # git  b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/HEAD
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/config
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/description
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/info/exclude
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/refs/heads/master
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/file
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/file with space + α
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/fileexec
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/symlink.dir
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/symlink.file
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/symlink.missing
        git-backup_test.go:203: tag f735011c9fcece41219729a33f7876cd8791f659 still present in backup.git after git-prune
    --- FAIL: TestPullRestore (0.14s)

Bisecting shows that particular behaviour of pulling into packs from test
repositories starts to happen after https://git.kernel.org/pub/scm/git/git.git/commit/?id=5476e1efded571e374cd97c7d69f17962ba1c44f .
But as explained in the beginning it is not a Git bug as in the general case just `git prune` is not enough to remove all unreachable objects.

-> Fix it by accompanying `git prune` with `git repack -ad` that removes unreachable objects from the packs.

NOTE just `git repack -ad` is not enough as it puts in packs only reachable
objects, so any unreachable objects that were loose before repacking, will
remain alive.

@lpgeneau reports that module-based build fails with

    2021-07-07 16:37:34 slapos[2415] INFO GOMODSRC /srv/slapgrid/slappart11/srv/runner/software/a12cdf3481c4202ae72cb255d8a6c183/go.work/src/lab.nexedi.com/kirr/git-backup:./...
    2021-07-07 16:37:34 slapos[2415] INFO ../../../../pkg/mod/lab.nexedi.com/kirr/go123@v0.0.0-20210302025843-863c4602a230/xerr/xerr.go:77:2: missing go.sum entry for module providing package github.com/pkg/errors (imported by lab.nexedi.com/kirr/go123/xerr); to add:
    2021-07-07 16:37:34 slapos[2415] INFO   go get lab.nexedi.com/kirr/go123/xerr@v0.0.0-20210302025843-863c4602a230
    2021-07-07 16:37:34 slapos[2415] INFO gowork.goinstall: Non zero exit code (1) while running command.

-> Fix it by updating go123 dependency and running recommended command to update go.sum.

Amends 3c804105 (*: Add Go modules support; Upgrade to latest git2go release)

If there is a folder in git repositories which ends with .git and which
is not a git repository, git-backup will fail because it will try to use
it as a git repository.

This patch just checks if it's really a git repo before continue inside.

[kirr: Reworked the patch a bit]
/reviewed-on: kirr/git-backup!8

@kirr This two commits are required to backup and restore with recent versions of gitlab.

- Skip gitlab page, else gitlab will try to restore it and fail.
- `Backup::Database.new` now require a parameter

/reviewed-on: kirr/git-backup!7

see: https://lab.nexedi.com/nexedi/gitlab-ce/blob/v11.11.8-nxd/lib/tasks/gitlab/backup.rake

Solve the problem:

    NoMethodError: undefined method `parse' for Time:Class

--------

kirr: it probaly started to fail due to gitlab codebase upgrade, as eariler gitlab-rake was likely requiring "time" and now does not.

/reviewed-by @kirr
/reviewed-on !6

So that further `git-backup pull` run can proceed, and many gitlab-backup's left `$tmpd` don't eat up disk space.

Rework of !4.

/reviewed-on !5

This lets deferred cleanup to be run instead of whole process being
killed without having a chance to preserve external data in consistent
state.

Add ctx parameter to cmd_pull, cmd_restore and inner functions that they
call that can block and handle ctx cancel where we can. For now both
pull and restore are always run under background context, but in the
next patch we'll connect SIGINT+SIGTERM to cancel spawned work.

In general a service - even a command line utility - needs to handle
cancellation properly and itself to maintain consistency of external
state. See e.g.

	https://callistaenterprise.se/blogg/teknik/2019/10/05/go-worker-cancellation/

for example.

The pattern where multiple workers are spawned to work on a common task
and where whole work needs to be canceled on first error is now well
understood, with the functionality to broadcast cancel and propagate
errors being wrapped into libraries such as

	https://godoc.org/golang.org/x/sync/errgroup			and
	https://godoc.org/lab.nexedi.com/kirr/go123/xsync#WorkGroup
	(kirr/go123@515a6d14)

Let's streamline the code by using xsync.WorkGroup (it is in our hands,
a bit more well designed (imho), has analog in Pygolang, and can be
changed/enhanced as needed).

The other reason to rework the code is that the workgroup is created
under context (currently always background) and can be canceled by that
context cancel. In the next patch we'll teach all git-backup
subcommands, including restore, to work under context, and by using
xsync.WorkGroup we will automatically handle cancellation from outside,
while without reworking extraction pipeline we would need to
additionally glue ctx cancel to signal to workers to stop.

Compared to previous state both xsync.WorkGroup and errogroup return
only the first error, however it should likely not cause problems in
practice as the first error is usually the most informative one.

Similarly to previous patch, let's cleanup gitlab-backup temporary
folder always unconditionally in the presence of errors. Keeping $tmpd
on error was not preventing further gitlab-backup run to proceed, but it
can quickly eat up disk space if there are many such runs. If debugging
is needed one can comment the cleanup, but by default let's be
production friendly out of the box.

Based on patch by @alain.takoudjou:
kirr/git-backup!4

Original description from Alain:

---- 8< ----
When script exit, remove tmp backup folder which are not longuer needed.
Keep this folder when backup is failing will contribute to fill the disk
of server. backup.locked is also removed, because we want to
automatically retry gitlab-backup if previous backup failed, without
human action. If the file is not removed automatically, backup is
blocked until someone remove it.

On pull git-backup locks backup repository to make sure another
concurrent `git-backup pull` process is not running. However until now,
if a pull was failing, the lock was left unreleased, which made followup
pull attempts to fail while acquiring the lock until the lock was
manually removed with `git update-ref -d ...`. Probably originally I
made it like this in 6f237f22 (git-backup: Initial draft) to make sure
that if there is a problem it does not go unnoticed and forces me to
investigate. But in general we do _not_ need to keep the lock on error
return after `git-backup pull` completes even abnormally.

This "lock left unreleased" is causing operational issues on
lab.nexedi.com from time to time: if a pull try fails for some, even
temporary, reason, all next pull tries will fail until a human intervene
and remove the lock ref.

Fix it.

See also: !4

	# lab.nexedi.com/kirr/git-backup
	./git.go:177: Raisef call needs 1 arg but has 2 args

The bug was there from day 1 after rewrite in Go in 28986e0e.

Without trailing dot the following sentence was included into TOC.

Like it was already said in 899103bf (pull: Switch from porcelain `git
fetch` to plumbing `git fetch-pack` + friends) currently on
lab.nexedi.com `git-backup pull` became slow and most of the slowness
was tracked down to the fact that `git fetch` for every pulled repository does
linear scan of whole backup repository history just to find out there is
usually nothing to fetch. Quoting 899103bf:

"""
    `git fetch`, before fetching data from remote repository, first checks
    whether it already locally has all the objects remote advertises. This
    boils down to running

	echo $remote_tips | git rev-list --quiet --objects --stdin --not --all

    and checking whether it succeeds or not:

	https://git.kernel.org/pub/scm/git/git.git/commit/?h=4191c35671
	https://git.kernel.org/pub/scm/git/git.git/tree/builtin/fetch.c?h=v2.18.0-rc1-1-g6f333ff2fb#n925
	https://git.kernel.org/pub/scm/git/git.git/tree/connected.c?h=v2.18.0-rc1-1-g6f333ff2fb#n8

    The "--not --all" in the query means that objects should be not
    reachable from all locally existing refs and is implemented by linearly
    scanning from tip of those existing refs and marking objects reachable
    from there as "do not print".

    In case of git-backup, where we have mostly master which is super commit
    merging from whole histories of all projects and from backup history,
    linearly scanning from such a tip goes through lots of commits. Up to
    the point where fetching a small, outdated repository, which was already
    pulled into backup and did not changed since long, takes more than 30
    seconds with almost 100% of that time being spent in quickfetch() only.
"""

The solution is that we can build index of objects we already have ourselves
only once at startup, and then in fetch, after checking lsremote output, consult
that index, and if we see we already have everything for an advertised
reference - just avoid giving it to fetch-pack to process. It turns out for
many pulled repositories there is no references changed at all and this way
fetch-pack can be skipped completely. This leads to dramatical speedup: before
`gitlab-backup pull` was taking ~ 2 hours, and now something under ~ 5 minutes.

The index building itself takes ~ 30 seconds - the time which we were
previously spending to fetch just from 1 unchanged repository. The index size
is small and so it all can be kept in RAM - please see details in the code
comments on this.

I initially wanted to speedup fetching by teaching `git fetch-objects` to
consult backup repo bitmap reachability index (if, for a commit, we can see
that there is an entry in this index -> we know we already have all reachable
objects for this commit and can skip fetching). This won't however work
fully for all our refs - 40% of them are mostly tags, and since in the backup
repository we don't keep tag objects - we keep tags/tree/blobs encoded as
commits - sha1 of those 40% references to tags won't be in bitmap index.

So just do the indexing ourselves.

In the next patch we will need to load backup.refs in the beginning of
pull too. Factored function changed to return regular error instead of
raising exception (which will be the general plan from now on).

On lab.nexedi.com `git-backup pull` became slow, and most of the slowness
was tracked down to the following:

`git fetch`, before fetching data from remote repository, first checks
whether it already locally has all the objects remote advertises. This
boils down to running

	echo $remote_tips | git rev-list --quiet --objects --stdin --not --all

and checking whether it succeeds or not:

	https://git.kernel.org/pub/scm/git/git.git/commit/?h=4191c35671
	https://git.kernel.org/pub/scm/git/git.git/tree/builtin/fetch.c?h=v2.18.0-rc1-1-g6f333ff2fb#n925
	https://git.kernel.org/pub/scm/git/git.git/tree/connected.c?h=v2.18.0-rc1-1-g6f333ff2fb#n8

The "--not --all" in the query means that objects should be not
reachable from all locally existing refs and is implemented by linearly
scanning from tip of those existing refs and marking objects reachable
from there as "do not print".

In case of git-backup, where we have mostly master which is super commit
merging from whole histories of all projects and from backup history,
linearly scanning from such a tip goes through lots of commits. Up to
the point where fetching a small, outdated repository, which was already
pulled into backup and did not changed since long, takes more than 30
seconds with almost 100% of that time being spent in quickfetch() only.

The solution will be to optimize checking whether we already have all the
remote objects and to not repeat whole backup-repo scanning for every
pulled repository. This will be done via first querying through `git
ls-remote` what tips remote repository has, then checking on
git-backup specific index which tips we already have and then fetching
only the rest. This way we are essentially moving most of quickfetch
phase of git into git-backup.

Since we'll be tailing to git to fetch only some of the remote refs, we
will either have to amend ourselves the refs `git fetch` creates after
fetching, or to not rely on `git fetch` creating any refs at all. Since
we already have a long standing issue that many many refs that are
coming live after `git fetch` slow down further git fetches

https://lab.nexedi.com/kirr/git-backup/blob/0ab7bbb6/git-backup.go#L551

the longer term plan will be not to create unneeded references.
Since 2 forks could have references covering the same commits, we would
either have to compare references created after git-fetch and deduplicate
them or manage references creation ourselves.

It is also generally better to split `git fetch` into steps at plumbing
layer, because after doing so, we can have the chance to optimize or
tweak any of the steps at our side with knowing full git-backup context
and indices.

This commit only switches from using `git fetch` to its plumbing
counterpart `git fetch-pack` + friends + manually creating fetched refs
the way `git fetch` used to do exactly. There should be neither
functionality changed nor any speedup.

Further commits will start to take advantage of the switch and optimize
`git-backup pull`.

- tell that reference name always goes without "refs/" prefix
- use .name for reference name, not .ref: this way

	ref.name

  is more readable than

	ref.ref

  and so there is less need to use for __ in range loops.

Noticed this while changing how pull works and making error there
incidentally with leaving more "refs/" prefix. With the error before
this patch tests show:

        git-backup_test.go:91: git-backup_test.go:204: lab.nexedi.com/kirr/git-backup.cmd_restore: 2 errors:
			- E: extracted /tmp/t-git-backup981909377/1/dir 2 + β/repo with+fragile name %αβγ.git refs corrupt:
			- E: extracted /tmp/t-git-backup981909377/1/dir/hello.git refs corrupt:

with the patch tests report:

        git-backup_test.go:91: git-backup_test.go:204: lab.nexedi.com/kirr/git-backup.cmd_restore: 2 errors:
                        - E: extracted /tmp/t-git-backup981909377/1/dir 2 + β/repo with+fragile name %αβγ.git refs corrupt:

                want:
                cbb6d3f205749888f77fb1a88fbac3b8a0b8000f refs/refs/heads/master

                have:
                cbb6d3f205749888f77fb1a88fbac3b8a0b8000f refs/heads/master
                        - E: extracted /tmp/t-git-backup981909377/1/dir/hello.git refs corrupt:

                want:
                647e137fd3b31939b36889eba854a298ef97b6ff refs/refs/heads/branch2
                feeed96ca75fcf8dcf183008f61dbf72e91ab4de refs/refs/heads/master
                11e67095628aa17b03436850e690faea3006c25d refs/refs/tags/tag-to-blob
                f735011c9fcece41219729a33f7876cd8791f659 refs/refs/tags/tag-to-commit
                7124713e403925bc772cd252b0dec099f3ced9c5 refs/refs/tags/tag-to-tag
                ba899e5639273a6fa4d50d684af8db1ae070351e refs/refs/tags/tag-to-tree
                7a3343f584218e973165d943d7c0af47a52ca477 refs/refs/test/ref-to-blob
                61882eb85774ed4401681d800bb9c638031375e2 refs/refs/test/ref-to-tree

                have:
                647e137fd3b31939b36889eba854a298ef97b6ff refs/heads/branch2
                feeed96ca75fcf8dcf183008f61dbf72e91ab4de refs/heads/master
                11e67095628aa17b03436850e690faea3006c25d refs/tags/tag-to-blob
                f735011c9fcece41219729a33f7876cd8791f659 refs/tags/tag-to-commit
                7124713e403925bc772cd252b0dec099f3ced9c5 refs/tags/tag-to-tag
                ba899e5639273a6fa4d50d684af8db1ae070351e refs/tags/tag-to-tree
                7a3343f584218e973165d943d7c0af47a52ca477 refs/test/ref-to-blob
                61882eb85774ed4401681d800bb9c638031375e2 refs/test/ref-to-tree

Should be good to have this details if something really breaks after restore.

This way, if backup repository was freshly repacked with bitmap index
generation turned on, we can get ~ 30% - 50% speedup for a typical
erp5.git pack extraction.

"--use-bitmap-index" option was added to git in v2.0, but was only
active for to-stdout packs generation. It was enabled for to-file packs
generation in git v2.11.

Since git v2.0 was released in 2014 - 4 years ago - I'm not adding
runtime detection of "--use-bitmap-index" availability.

See https://git.kernel.org/pub/scm/git/git.git/commit/?h=645c432d61 for
details.

- remove blank line between main description and package clause, so that
  the main description is understood as such;
- move notes describing what a file does after package clause, so that
  those notes do not get mixed into program description under godoc.

add option to remove or keep pulled backup data

[ kirr: The .pulled files with gitlab backup data (SQL and the like)
  were originally not removed "just in case" in the early days of
  git/gitlab-backup. They are clearly not needed to be kept since their
  content is entered into git backup database by gitlab-backup, and
  leaving those .pulled files just wastes disk space.

  So default to not keep them around and for now add an option to
  forcibly preserve the raw gitlab backup if we'll need it just in case or
  for the debugging.

  However if it turns out we won't really need -keep in practice, it
  might go away in some time. ]

/reviewed-on !3

If a repository is removed when git-backup is running, print a warning
message and continue pulling instead of exiting with error.

/reviewed-on kirr/git-backup!2

Relicense to GPLv3+ with wide exception for all Free Software / Open Source projects + Business options.

Nexedi stack is licensed under Free Software licenses with various exceptions
that cover three business cases:

- Free Software
- Proprietary Software
- Rebranding

As long as one intends to develop Free Software based on Nexedi stack, no
license cost is involved. Developing proprietary software based on Nexedi stack
may require a proprietary exception license. Rebranding Nexedi stack is
prohibited unless rebranding license is acquired.

Through this licensing approach, Nexedi expects to encourage Free Software
development without restrictions and at the same time create a framework for
proprietary software to contribute to the long term sustainability of the
Nexedi stack.

Please see https://www.nexedi.com/licensing for details, rationale and options.

- myname moved -> my
  kirr/go123@98249b24

- Traceback now returns []runtime.Frame
  kirr/go123@7deb28a5

to

	xflag.Count

	xstrings.SplitLines
	xstrings.Split2
	xstrings.HeadTail

Other string-related routines stay in git-backup for now as I don't
feel they are general enough or interface chosen is really ok.

It is now

	mem.String(),	and
	mem.Bytes()

error.go is completely being moved to that shared place for handy Go
utilities into several subpackages:

	lab.nexedi.com/kirr/go123/exc		-- exception-style error handling for Go
	lab.nexedi.com/kirr/go123/myname	-- easy way to determine current function's name and package
	lab.nexedi.com/kirr/go123/xerr		-- addons for error-handling
	lab.nexedi.com/kirr/go123/xruntime	-- addons to standard package runtime

By definition of strings.Split(..., sep) it "slices s into all substrings
separated by sep and returns a slice of the substrings between those
separators". That means that

    string.Split("hello\nworld\n", "\n") -> ["hello", "world", ""])     # NOTE the last ""

when parsing file by lines, it is handy though to do not get last empty
"" after last "\n". #6 shows how we missed to do that filtering-out for
case of empty backup.refs file and errored-out because of that.

To fix let's introduce a helper - splitlines(), which does the job of
filtering-out last empty entry after last separator. By using this
helper everywhere we can hopefully avoid problems while pulling only
empty repositories (#6 case), and also similar ones.

Fixes #6
/reported-by @iv

Continuing 62374038 (pull: Turns unused refs are removed not 100% and a
lot of empty directories are accumulated) we just make sure to remove
them in the end of pull.

But NOTE: there could be O(n^2) behaviour still hidden, so it makes
sense to eventually revisit it and cleanup empty dirs earlier.

For now we just care not to degrade future pull performance. The
appropriate time for revisiting could be when reworking pull to do
fetches in parallel.

Updates: https://lab.nexedi.com/lab.nexedi.com/lab.nexedi.com/issues/4

This way it allows us to leverage multiple CPUs on a system for pack
extractions, which are computation-heavy operations.

The way to do is more-or-less classical:

    - main worker prepares requests for pack extraction jobs

    - there are multiple pack-extraction workers, which read requests
      from jobs queue and perform them

    - at the end we wait for everything to stop, collect errors and
      optionally signalling the whole thing to cancel if we see an error
      coming. (it is only a signal and we still have to wait for
      everything to stop)

The default number of workers is N(CPU) on the system - because we spawn
separate `git pack-objects ...` for every request.

We also now explicitly limit N(CPU) each `git pack-objects ...` can use
to 1. This way control how many resources to use is in git-backup hand
and also git packs better this way (when only using 1 thread) because
when deltifying all objects are considered to each other, not only all
objects inside 1 thread's object poll, and even when pack.threads is not
1, first "objects counting" phase of pack is serial - wasting all but 1
core.

On lab.nexedi.com we already use pack.threads=1 by default in global
gitconfig, but the above change is for code to be universal.

Time to restore nexedi/ from lab.nexedi.com backup:

2CPU laptop:

    before (pack.threads=1)     10m11s
    before (pack.threads=NCPU)   9m13s
    after  -j1                  10m11s
    after                        6m17s

8CPU system (with other load present, noisy) :

    before (pack.threads=1)     ~5m
    after                       ~1m30s

like in 302aaaea (raiseif: Fix it wrt erraddcallingcontext()) now fix
raisef, which I originally overlooked.