This restores the "Revert Changes.." functionality
that was removed when changing the file list stuff.
It's now possible to revert multiple files. Also, it
now uses 'git checkout-index', which should be a bit
more robust.
We previously used some cool git syntax to display changes in the index.
The unfortunate side-effect of this was that the diff headers weren't correct,
so the unstage button didn't work anymore.
This merges functionality that was previously stored in the
combination of PBGitCommitController / PBChangedFile to a
dedicated controller, PBGitIndexController.