Add "Author" and "Parent Revision" hardpoints to RevisionRefs

Summary: Ref T13546. These are used by a future "arc land" workflow to support the "Land changes you don't own?" and "Land changes with open dependencies?" prompts.

Test Plan: Ran a future "arc land" flow, hit both prompts.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21311