Diffusion - move history query to conduit

Summary: Ref T2784

Test Plan: for each flavor of VCS, I loaded up the repository home page. verified I saw some parent action where appropos. next, clicked through to 'view history' and verified it loaded up A-OK.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5960