In Herald "Commit" rules, use repository identities to identify authors and committers

Summary: Ref T13480. The Herald "Commit" rules still use raw commit data properties to identify authors and committers. Instead, use repository identities.

Test Plan: Wrote a Herald rule using all four fields, ran it against various commits with and without known authors. Checked transcript for sensible field values.

Maniphest Tasks: T13480

Differential Revision: https://secure.phabricator.com/D20955