Expose commit information via conduit instead of user information

Summary:
After D4825, this information is often available to us in a safe way. Provide it explictly.

This removes or reduces functionality in some cases, but I think we can plug those holes with Conpherence addresses and/or explicit user acknowledgement/config.

Test Plan: Patched a commit with `arc patch` and got the original address out.

Reviewers: btrahan, edward, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4828