When landing revisions via repository automation, use better metadata

Summary: Ref T182. Make a reasonable attempt to get the commit message, author, and committer data correct.

Test Plan: BEHOLD: rGITTEST810b7f17cd0c909256a45d29a5062fcf417d0489

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14280