Because not only go.git wants to increase verbosity on `-v -v ...`.

Code taken from go.git ec80737bdf.

No many details since they can be all viewed e.g. with help of godoc.
Just a top-level pointer.

Move them here to go123 like in previous patches. Taken from git-backup
0be1f647.

[1] https://lab.nexedi.com/kirr/git-backup

Pristine import + package/copyright clauses. Taken from git-backup 3aedc246.

[1] https://lab.nexedi.com/kirr/git-backup

Export functions that client would use and adjust their names taking
into account that now there is a package prefix. So e.g.

	raise		-> exc.Raise
	errcatch	-> exc.Catch
	erronunwind	-> exc.Onunwind
	...

It is completely orthogonal to exception handling.

It is

	xruntime.Traceback()

Usage would be:

	myname.Func(),	and
	myname.Pkg()

This is pristine import. Files are taken as-is from git-backup 3ba6cf73.
Will adapt them to live in separate package in the next commit.

[1] https://lab.nexedi.com/kirr/git-backup

This will be a place to keep handy go bits shared across projects.