When a commit has a "rewritten" hint, show it in the UI instead of the generic "deleted" message

Summary:
Ref T11522. When a commit is no longer reachable from any branch/tag, we currently show a "this has been deleted" message.

Instead, go further: check if there is a "rewritten" hint pointing at a commit the current commit was rewritten into. If we find one, show a message about that instead.

(This isn't super pretty, just getting it working for now. I expect to revisit this UI in T9713 if we don't get to it before that.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11522

Differential Revision: https://secure.phabricator.com/D16436