R1:0014d0404c2d

Consolidate repository identity resolution and detection code

Summary: Ref T13444. Send all repository identity/detection through a new "DiffusionRepositoryIdentityEngine" which handles resolution and detection updates in one place.

Test Plan:
- Ran `bin/repository reparse --message ...`, saw author/committer identity updates.
- Added "goose@example.com" to my email addresses, ran daemons, saw the identity relationship get picked up.
- Ran `bin/repository rebuild-identities ...`, saw sensible…