Add "raw author name" and "raw committer name" as Herald fields for commit content hooks

Summary:
Ref T4195. A legitimate rule which needs this field is "do not allow commits as root". Interestingly, we have exactly one commit as root in each Phabricator, Arcanist and libphutil.

Since the committer and author don't need to be Phabricator accounts (just the Pusher), the existing "Committer" and "Author" fields can't express this rule (they'll be empty).

Test Plan: {F93406}

Reviewers: btrahan

Reviewed By: btrahan

CC: SEJeff, aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7841