R1:514ee3526c3d

Add an event for looking up names from repositories

Summary:
Currently, we have this cumbersome `PhabricatorRepositoryCommitMessageDetailParser` hook. This is really old and outdated; I want to just use the Differential custom field parser. See T945 for a specific application.

However, it allows installs to override author/committer association. Instead, provide an event hook for doing this.

Test Plan: Added a listener, made every commit resolve to "turtle", parsed some commits, verified the events…