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