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.

• Pushed a commit as xyz <xyz@example.org>, an address with no corresponding user.
• In the UI, manually associated that identity with user @alice.
• Ran bin/repository reparse --publish <hash> to trigger audits and publishing for the commit.
• Before: observed the $author_phid was null.
• After: observed the $author_phid is Alice.