Probably a good candidate for building on {T1703}.

This problem potentially has very large scope -- at Facebook, many users (particularly ezra and mcslee) had dozens of VCS aliases (primarily from manual "Reviewed By" over a long period of time) and it would have been unreasonable to get a high-quality association between commits and authors by having users manually pick VCS names. Even I had a dozen or so because my last name is hard to spell / easy to typo. Phabricator can also track multiple repositories, and the same user might have different names in different repositories (and different users might have the same name in separate repositories, for that matter).

I think these hard cases are already well-handled by the PhabricatorEventType::TYPE_DIFFUSION_LOOKUPUSER event (which gives you all the information you need to go look up the answer in an external datasource), and I think we do the best job we can in the easy cases where users have given us enough information that we can plausibly guess who is who. But the cases in between, where there are a smaller number of more consistent usernames and no one wants to build an event listener and manually build/curate the alias list aren't well served right now.

Another other problem is that commits are associated with authors at parse/import time, so setting a VCS name will work going forward but not retroactively. Administrators can fix retroactive commits with ./scripts/repository/reparse.php --all --message X, where X is the callsign, but this is bound to cause some confusion (similar to how people show up somewhat regularly with confusion about Differential caches after enabling Pygments). I don't think we necessarily need to make this "just work", but should provide messaging so expectations are clear.