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.
- Ran bin/repository reparse --message ..., saw author/committer identity updates.
- Added "firstname.lastname@example.org" to my email addresses, ran daemons, saw the identity relationship get picked up.
- Ran bin/repository rebuild-identities ..., saw sensible rebuilds.
Maniphest Tasks: T13444
Differential Revision: https://secure.phabricator.com/D20910