This adds a slider in the commit view with
which the user can change the context size.
This is useful for example if the hunks are too
big; by changing the context size, a hunk can be
split and then the changes can be committed.
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.