Summary: Ref T2222. Currently, Differential has a fairly hairy piece of logic to parse object lists, like `Reviewers: alincoln, htaft`. Extract, generalize, and cover this. - Some of the logic can be simplified with modern ObjectQuery stuff. - Make `@username` the formal monogram for users. - Make `list@domain.com` the formal monogram for mailing lists. - Add test coverage. Test Plan: - Ran unit tests. - Called `differential.parsecommitmessage` with a bunch of real-world inputs and got sensible results. Reviewers: btrahan Reviewed By: btrahan CC: aran Maniphest Tasks: T2222 Differential Revision: https://secure.phabricator.com/D8445