tools: a simple git.mergetool for working with buildout.hash.cfg

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.