Add script to delete revisions without diff

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