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 This is really old and outdated; I want to just use the Differential custom field parser.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 looked sane and they now correctly were all attributed to "turtle".

Reviewers: btrahan, vrana, nh

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1337

Differential Revision: https://secure.phabricator.com/D3040