R1:7fd6bf26a949

Modernize "Author" and "Committer" rendering for commits

Summary:
Ref T13552. Give "Commit" objects a more modern, identity-aware way to render author and committer information.

This uses handles in a more modern way and gives us a single read callsite for raw author and committer names.

Test Plan:
- Grepped for callers to the old methods, found none. (There are a lot of "renderAuthor()" callers in transactions, but this call takes no parameters.)
- Viewed some commits, saw sensible lists of authors…