Don't put bad mailing list names in the CC list of a commit message

Summary:
If we can't look up the name for a mailing list, don't put 'Unknown Mailing
List' in the commit message - it will confuse things later down the road.

Surely there is a better way of doing this than checking the name of the
handle for the mailing list.

Test Plan:
called differential.getcommitmessage on a revision that had an invalid mailing
list phid.

Reviewers: epriestley, schrockn, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1910