Provide a script to "undo" the negative effects of an accidental push in Differential

Summary: If someone accidentally pushes a bunch of commits, revisions might get marked as "Committed" incorrectly. This will restore them to their previous state without too much fuss.

Test Plan: Ran the script on some commits to undo them, it seemed to work correctly.

Reviewers: davidreuss, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1877