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

src/applications/diffusion/controller/DiffusionController.php

@@ -321,4 +321,17 @@ abstract class DiffusionController extends PhabricatorController {
     return $crumb_list;
   }
 
+  protected function callConduitWithDiffusionRequest(
+    $method,
+    array $params = array()) {
+
+    $user = $this->getRequest()->getUser();
+    $drequest = $this->getDiffusionRequest();
+
+    return DiffusionQuery::callConduitWithDiffusionRequest(
+      $user,
+      $drequest,
+      $method,
+      $params);
+  }
 }