Mostly modularize the Differential "update" transaction

Summary: Ref T13099. Move most of the "Update" logic to modular transactions

Test Plan: Created and updated revisions. Flushed the task queue. Grepped for `TYPE_UPDATE`. Reviewed update transactions in the timeline and feed.

Maniphest Tasks: T13099

Differential Revision: https://secure.phabricator.com/D19175

resources/sql/autopatches/20140212.dx.1.armageddon.php

@@ -75,7 +75,7 @@ foreach ($rows as $row) {
 
   if ($diff_id || $row['action'] == DifferentialAction::ACTION_UPDATE) {
     $xactions[] = array(
-      'type' => DifferentialTransaction::TYPE_UPDATE,
+      'type' => DifferentialRevisionUpdateTransaction::TRANSACTIONTYPE,
       'old' => null,
       'new' => $diff_id,
     );