R1:e454c3dafe99

Wrap all direct access to author/committer properties on "CommitData"

Summary: Ref T13552. Currently, various callers read raw properties off "CommitData" directly. Wrap these in accessors to support storage changes which persist "CommitRef" information instead.

Test Plan:
- Ran "diffusion.querycommits", saw the same data before and after.
- Looked at a commit, saw authorship information and date.
- Viewed tags in a repository, saw author information.
- Ran "rebuild-identities", saw no net effect.