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.
• Grepped for callers to "getCommitDetail(...)".

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21448