Convert a low-level VCS query in Diff extraction to a Conduit call

Summary: Ref T9319. Ref T2783. This won't currently work in a future environment where daemons and repositories are not on the same host. Send it over Conduit instead.

Test Plan: Used `bin/differential attach-commit rX Dy` to force attachment, saw valid content pull over Conduit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2783, T9319

Differential Revision: https://secure.phabricator.com/D14969

src/applications/differential/engine/DifferentialDiffExtractionEngine.php

@@ -142,17 +142,21 @@ final class DifferentialDiffExtractionEngine extends Phobject {
           return true;
         }
 
-        $drequest = DiffusionRequest::newFromDictionary(array(
+        $drequest = DiffusionRequest::newFromDictionary(
+          array(
             'user' => $viewer,
             'repository' => $repository,
+          ));
+
+        $response = DiffusionQuery::callConduitWithDiffusionRequest(
+          $viewer,
+          $drequest,
+          'diffusion.filecontentquery',
+          array(
             'commit' => $identifier,
             'path' => $path,
           ));
-
-        $corpus = DiffusionFileContentQuery::newFromDiffusionRequest($drequest)
-          ->setViewer(PhabricatorUser::getOmnipotentUser())
-          ->loadFileContent()
-          ->getCorpus();
+        $corpus = $response['corpus'];
 
         if ($files[$file_phid]->loadFileData() != $corpus) {
           return true;