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 occasionally some confusion over how author/committer strings resolve into valid users.