Make "CommitData" wrap and persist a "CommitRef" record

Summary:
Ref T13552. Turn "CommitData" into an application-level layer on top of the repository-level "CommitRef" object.

For older commits which will not have a "CommitRef" record on disk, build a synthetic one at runtime. This could eventually be migrated.

Test Plan: Ran "bin/repository reparse --message", browsed Diffusion.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21449