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