Differential - tighten up access of Differential data from other applications

Summary: Fixes T6790. Turn the old method into "new" (old signature) and "newEphemeral". Deploy "newEphemeral" as many places as possible; basically places we are not in the Differential application *and* have no intentions of ever saving the diff. These callsites are also all places we are just trying to get some changesets at the end of the day.

Test Plan: set differential application policy to 'administrators only'. viewed a commit in diffusion and it worked without any errors! i'm just using my thinkin' noodle on the other code paths.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6790

Differential Revision: https://secure.phabricator.com/D11020

src/infrastructure/diff/PhabricatorDifferenceEngine.php

@@ -163,8 +163,7 @@ final class PhabricatorDifferenceEngine {
     $diff = $this->generateRawDiffFromFileContent($old, $new);
 
     $changes = id(new ArcanistDiffParser())->parseDiff($diff);
-    $diff = DifferentialDiff::newFromRawChanges(
-      PhabricatorUser::getOmnipotentUser(),
+    $diff = DifferentialDiff::newEphemeralFromRawChanges(
       $changes);
     return head($diff->getChangesets());
   }