Respect repository identities when selecting author vs auditor actions

Summary:
Depends on D20580. Fixes T13311. When we choose which actions to show a user, we can either show them "auditor" actions (like "raise concern") or "author" actions (like "request verification").

Currently, we don't show "author" actions if you're the author of the commit via an identity mapping, but we should. Use identity mappings where they exist.

(Because I've implemented getEffectiveAuthorPHID() in a way that requires $data be attached, it's possible this will make something throw a "DataNotAttached" exception, but: probably it won't?; and that's easy to fix if it happens.)

Test Plan:
See D20580. As @alice, viewed the commit in the UI.

• Before: got auditor actions presented to me.
• After: got author actions.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13311

Differential Revision: https://secure.phabricator.com/D20581