Add "pusher is committer" to Herald as a pre-commit rule

Summary:
Fixes T4594. Also, allow "exists" / "does not exist" to be run against author/committer. This allows construction of rules like:

• Committer identities must be authentic.
• Committer identities must be resolvable.
• Author identities must be resolvable.

Test Plan: Created some rules using these new rules and ran them.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4594

Differential Revision: https://secure.phabricator.com/D8507