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.

• 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.