Diffusion - move DiffQuery to Conduit

Summary: title does not say it all; also added conduit wrappers for rawdiffquery and lastmodifiedquery. These get the wrapper treatment since they are used in daemons. Ref T2784.

Test Plan: for each of the 3 VCS, did the following: 1) loaded up CALLSIGN and verified 'Modified' column data showed up correctly in Browse Repository box. 2) loaded up the "change" view for a specific file and verified content showed up correctly. 3) loaded up a specific commit and noted the changes ajax loaded A-OK

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5896