R1:c79d26144a01

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…