Add support for 'rbt patch -c' for Mercurial repositories

Implement create_commit() in mercurial.py to support committing using
information from the review request.

Testing Done:
Posted a patch to Review Board, popped it off the queue, ran 'rbt patch -c
<review_id>' and verified that the changeset was correctly committed.

Reviewed at https://reviews.reviewboard.org/r/6090/