Add "Author" and "Committer" fields to Herald pre-commit content hooks
Summary: Ref T4195. Adds "Author" and "Committer" fields.
Created a rule using these fields:
...then pushed git, mercurial and svn commits and verified the correct values populated in the transcript:
Reviewed By: btrahan
Maniphest Tasks: T4195
Differential Revision: https://secure.phabricator.com/D7802