Populate results of DiffusionQueryCommitsConduitAPIMethod with DiffusionLowLevelCommitQuery

Summary:
Ref T2783.  This populates the following fields in DiffusionQueryCommitsConduitAPIMethod using DiffusionLowLevelCommitQuery when `bypassCache` is set to true:

  * `authorName`
  * `authorEmail`
  * `committerName`
  * `committerEmail`
  * `message`
  * `hashes`

The original outline called for `authorPHID` and `committerPHID` as well (but no `message` field).  As far as I can tell, the PHIDs aren't actual a property on `DiffusionCommitRef`, and since the intention of this is to be able to populate a `DiffusionCommitRef`, I haven't included them.  Let me know if we really do need the PHIDs here.

Test Plan: Tested using 3 Phabricator instances (one web, one taskmaster and one storage).  The web and taskmaster tiers are directed at the Conduit API of the storage tier.  Made a `diffusion.querycommits` from the Conduit app on the web tier instance and saw the data populated from the raw VCS data (located on the storage tier).

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D10399