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

src/applications/differential/controller/DifferentialRevisionViewController.php

@@ -881,11 +881,6 @@ final class DifferentialRevisionViewController extends DifferentialController {
       ->setRightDiff($right_diff)
       ->setTransactions($xactions);
 
-    // TODO: Make this work and restore edit links. We need to copy
-    // `revisionPHID` to the new version of the comment. This should be simple,
-    // but can happen post-merge. See T2222.
-    $timeline->setShowEditActions(false);
-
     return $timeline;
   }