Rewriting commit history with `git rebase -i` always caused some
conflicts on md5sum of modified files. `update-hash` made all this
easier, because we could just revert buildout.hash.cfg, re-run
`update-hash` and commit the changes.

This tool makes this scenario a bit more user friendly by automating the
steps by running as a git mergetool.

An interactive rebase session can be initiated by running this command
from software/*/ directory:

`git rebase -i --exec "$(pwd)/../../update-hash $(pwd)/buildout.hash.cfg" origin/master`

and if conflict occur, run:

`git mergetool --tool update-hash-mergetool && EDITOR=cat git rebase --continue`

This way, rebase session can be almost non interactive (still have to
run the above mergetool command) when conflicts are only on md5sum
updates.

/reviewed-on nexedi/slapos!273