git-gui learned to revert selected lines and hunks, just like it can
stage selected lines and hunks. To provide a safety net for accidental
revert, the most recent revert can be undone.
* py/revert-hunks-lines:
git-gui: allow undoing last revert
git-gui: return early when patch fails to apply
git-gui: allow reverting selected hunk
git-gui: allow reverting selected lines