Allow discovered but unparsed commits to be requested in Releeph

Summary:
Ref T3662. Releeph blocks users from requsting unparsed commits, but there's no real technical reason for this.

The releephwork.getorigcommitmessage method assumes data exists, but should be replaced with diffusion.querycommits anyway.

Test Plan: Ran diffusion.querycommits. Requested a commit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3662

Differential Revision: https://secure.phabricator.com/D8823