Make JOIN changes to CommitQuery only

Summary:
Fixes T4911. See D8879. This gives us the correct query in cases where there are no audits.

This doesn't try to do the GROUP BY stuff yet.

Test Plan:

• Viewed a commit in Diffusion with no audits, got a commit detail page.
• Viewed "All Commits" in web UI, saw commits without any audits included in the list.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4911

Differential Revision: https://secure.phabricator.com/D8882