Diffusion - break out readme query all on its own

Summary: nice title. Ref T2784. Fixes T3171.

Test Plan: For all 3 VCS types, viewed phabricator repository and saw readme. browsed said repository and saw readme.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2784, T3171

Differential Revision: https://secure.phabricator.com/D5993