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 and committers.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21405