Fix appearance of "Differential Revision" on edit interfaces

Summary: The recent change to the field causes us to render "http://junk.com/D"
in some cases, just null the field if there's no data.

Test Plan: Ran "arc diff --create".

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1321

src/applications/differential/field/specification/revisionid/DifferentialRevisionIDFieldSpecification.php

@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -47,6 +47,9 @@ final class DifferentialRevisionIDFieldSpecification
   }
 
   public function renderValueForCommitMessage($is_edit) {
+    if ($is_edit || !$this->id) {
+      return null;
+    }
     return PhabricatorEnv::getProductionURI('/D'.$this->id);
   }