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/query/DiffusionQuery.php

@@ -36,6 +36,27 @@ abstract class DiffusionQuery extends PhabricatorQuery {
     return $this->request;
   }
 
+  final public static function callConduitWithDiffusionRequest(
+    PhabricatorUser $user,
+    DiffusionRequest $drequest,
+    $method,
+    array $params = array()) {
+
+    $repository = $drequest->getRepository();
+
+    $core_params = array(
+      'callsign' => $repository->getCallsign()
+    );
+    $params = $params + $core_params;
+
+    return id(new ConduitCall(
+      $method,
+      $params
+    ))
+    ->setUser($user)
+    ->execute();
+  }
+
   public function execute() {
     return $this->executeQuery();
   }