Summary: detect all revisions that don't have a diff, then delete them.
Test Plan:
we have been using this script for several months in
Facebook and it's working well.
Reviewers: epriestley, nh, btrahan
Reviewed By: epriestley
CC: aran, epriestley
Maniphest Tasks: T605
Differential Revision: https://secure.phabricator.com/D2061
