Always provide a valid content source for DifferentialCommentEditor

Summary: Ref T2222. On the `tmp.differential` branch, we're currently having
issues parsing commits which reference Differential revisions, because the
"user closed this revision (closed by commit xyz)" message is fataling:

  [2014-02-13 14:12:36] EXCEPTION: (PhutilProxyException) Error while
  executing task ID 345358 from queue. {>} (AphrontQueryException)
  #1048: Column 'contentSource' cannot be null

Specifically, the MessageParser pathway for CommentEditor doesn't set a content
source. Make sure CommentEditor always sets a content source.

(This is also causing a buildup of diffs on D8212 and D8211.)

Auditors: btrahan

src/applications/differential/editor/DifferentialCommentEditor.php

@@ -561,9 +561,15 @@ final class DifferentialCommentEditor extends PhabricatorEditor {
       ->setRevision($revision);
 
     if ($this->contentSource) {
-      $template->setContentSource($this->contentSource);
+      $content_source = $this->contentSource;
+    } else {
+      $content_source = PhabricatorContentSource::newForSource(
+        PhabricatorContentSource::SOURCE_LEGACY,
+        array());
     }
 
+    $template->setContentSource($content_source);
+
     $comments = array();
 
     // If this edit performs a meaningful action, save a transaction for the