Allow the new Differential EditEngine form to create/update diffs for revisions

Summary: Ref T11114. Much of this is around making the "comment-while-updating" flow work correctly.

Test Plan:

• Created new diffs by copy/pasting, then:
  • used one to create a new revision;
  • used one to update an existing revision, with a comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17053