Summary: Fixes T12251. Ref T13189. See PHI610. The difficulty here is that we don't want to disclose Phabricator account information to Buildkite. We're comfortable disclosing information from `git`, etc. - For commits, use the Identity to provide authorship information from Git. - For revisions, use the local commit information on the Diff to provide the Git/Mercurial/etc author of the HEAD commit. Test Plan: - Built commits and revisions in Buildkite via Harbormaster. - I can't actually figure out how to see author information on the Buildkite side, but the values look sane when dumped locally. Reviewers: amckinley Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam Maniphest Tasks: T13189, T12251 Differential Revision: https://secure.phabricator.com/D19614