Show repository information (and use repository identities) in commit hovercards

Summary:
Ref T12164. Ref T13439. Commit hovercards don't currently show the repository. Although this is sometimes obvious from context, it isn't at other times and it's clearly useful/important.

Also, use identities to render author/committer information and show committer if the committer differs from the author.

Test Plan: {F6989595}

Maniphest Tasks: T13439, T12164

Differential Revision: https://secure.phabricator.com/D20881