R1:0dc73d5d9357

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…