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