DiffusionFileContentQuery => Conduit

Summary: Ref T2784. This is probably pretty good except the fancy lint error saver now issue serial queries via Conduit.

Test Plan: reparsed commits on 3 repos - yay. viewed readme from diffusion UI on 3 repos - yay. viewed file content from diffusion UI on 3 repos - yay.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5824