Make "EditPro" controller work with diff updates

Summary:
Ref T2222. Make the "EditPro" controller accommodate diff updates, and support the transaction type. This one is pretty straightforward.

Also make `revisionPHID` in the comments table nullable to fix the "Edit" action.

Test Plan:
  - Created new revision.
  - Updated revision.
  - Tried to do some invalid stuff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8376