Return repositoryPHID from differential.query

Test Plan: Queried a revision that had a repository attached, got the PHID; queried one that didn't, got null.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9928