R1:632cafec881b

Pass commit authorship information to Buildkite

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…