Only trigger reviews on new diff revisions

Review Request #3682 — Created Dec. 18, 2012 and submitted — Latest diff uploaded

Information

ReviewBot

Reviewers

Only trigger reviews on new diff revisions.

Review Bot was firing requests to the review tools on any update to
a review request. Until proper support is added for reviewing things
other than the diff itself, we'll only fire off requests when a new
diff revision is uploaded.
Ran Review Bot with review request updates with and without new diffs, saw the expected behaviour.