Convert differential.revisionPHID commit detail to edge

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