Summary: Migration doesn't delete differential.revisionPHID but maybe it should?
Test Plan: Reparsed commit, ran the migration, deleted differential.revisionPHID, looked at task with attached commit with attached revision.
Reviewers: epriestley, edward
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Differential Revision: https://secure.phabricator.com/D5634
