R1:4450c908818d

When triggering audits, respect committer identities when importing commits

Summary:
Ref T13311. We currently don't use committer identity mappings when triggering audits, so if a user is only associated with an identity via manual mapping we won't treat them as the author.

Instead, use the identity and manual mapping if they're available.

Test Plan:
- Pushed a commit as `xyz <xyz@example.org>`, an address with no corresponding user.
- In the UI, manually associated that identity with user `@alice`.