Update the revert command and things relating to revert

Update the revert command and things relating to revert

Cherry-pick has been fixed, but even though revert does
basically the same thing, the fixes were not carried over here.

- Recognize the revert-states, analogous to the cherry picking states
- Make reset handle a revert-in-progress
- Update REVERT_HEAD and MERGE_MSG when revert fails due to conflicts
- Clear revert state on commit and reset
- Format the message similarily to how cherry-pick does. This is
  not exactly how C Git does it.

The interface is still not the same as for cherry-picking.

Change-Id: I8ea956fcbc9526d62a2365360feea23a9280eba3
Signed-off-by: Chris Aniszczyk <zx@twitter.com>