Add GROUP BY to commit query

Summary:
Ref T4715. Some minor stuff I caught locally while poking around:

  - Since we don't `GROUP BY`, we can still get duplicate commits. These get silently de-duplicated by `loadAllFromArray()` because that returns an array keyed by `id`, but we fetch too much data and this can cause us to execute too many queries to fill pages. Instead, `GROUP BY` if we joined the audit table.
  - After adding `GROUP BY`, getting the audit IDs out of the query is no longer reliable. Instead, query audits by the commit PHIDs. This is approximately equiavlent.
  - Since we always `JOIN`, we currently never return commits that don't have any audits. If we don't know that all results will have an audit, just `LEFT JOIN`.
  - Add some `!== null` to catch the `withIDs(array())` issue that we hit with Khan Academy a little while ago.

Test Plan:
  - Verified that "All Commits" shows commits with no audits of any kind.
  - Verified that the raw data comes out of the query without duplicates.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5433, T4715

Differential Revision: https://secure.phabricator.com/D8879