R1:d667b1220662

Provide a standalone query for resolution of commit author/committer into Phabricator users

Summary:
Ref T4195. To implement the "Author" and "Committer" rules, I need to resolve author/committer strings into Phabricator users.

The code to do this is currently buried in the daemons. Extract it into a standalone query.

I also added `bin/repository lookup-users <commit>` to test this query, both to improve confidence I'm getting this right and to provide a diagnostic command for users, since there's…