Diffusion - move DiffusionExistsQuery to work over conduit

Summary: le title. However, this also "is the first" conversion so sets the precedence for how this will all work. See comments in the code. I think this helps us keep the new code we're writing to a minimum. Wondering if the conduit end point could be more generic, and rather than have a switch statement on VCS type, one can just implement the "handleSubversion" version and have that called? Ref T2784

Test Plan: slapped an "or true" in the conditional protecting this code path. verified it worked on all 3 vcs systems, including typing in garbage and getting a 404

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5803