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 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