Allow differential.query to find accepted and committed revisions; fix a fatal

Summary:

• Expose existing 'committed' filter.
• Add an 'accepted' filter.
• Fix a fatal where $repository may not be defined (for diffs not linked to a

repository).

Test Plan: Ran accepted / committed queries. Viewed a previously fataling diff.

Reviewers: btrahan, vrana, Makinde

Reviewed By: Makinde

CC: Koolvin, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1490