Use modern two-stage markup cache (PhabricatorMarkupInterface) in Differential

Summary:
See T1963 for discussion of the Facebook-specific hack.

Differential currently uses a one-stage cache (render -> postprocess -> save in cache) rather than the two-stage cache (render -> save in cache -> postprocess) offered by `PhabricatorMarkupInteface`. This breaks Differential comments coming out of cache for the lightbox, and makes various other things suboptimal (status of handles like @mentions and embeds are not displayed accurately).

Instead, use the modern stuff.

Test Plan:
  - Created preview comments and inlines in Differential.
  - Edited a Differential inline.
  - Submitted main and inline Differential comments.
  - Viewed and edited Differential summary and test plan.
  - Created preview comments and inlines in Diffusion.
  - Submitted comments and inlines in Diffusion.
  - Verified Differential now loads and saves to the generalized markup cache (Diffusion is close, but main comments still hold a single-stage cache).
  - Verified old Differential comments work correctly with the lightbox.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1963

Differential Revision: https://secure.phabricator.com/D3804

src/applications/diffusion/controller/DiffusionDiffController.php

@@ -64,12 +64,11 @@ final class DiffusionDiffController extends DiffusionController {
       return new Aphront404Response();
     }
 
+
     $parser = new DifferentialChangesetParser();
     $parser->setUser($user);
     $parser->setChangeset($changeset);
     $parser->setRenderingReference($diff_query->getRenderingReference());
-    $parser->setMarkupEngine(
-      PhabricatorMarkupEngine::newDiffusionMarkupEngine());
 
     $pquery = new DiffusionPathIDQuery(array($changeset->getFilename()));
     $ids = $pquery->loadPathIDs();
@@ -98,6 +97,19 @@ final class DiffusionDiffController extends DiffusionController {
       $parser->setHandles($handles);
     }
 
+    $engine = new PhabricatorMarkupEngine();
+    $engine->setViewer($user);
+
+    foreach ($inlines as $inline) {
+      $engine->addObject(
+        $inline,
+        PhabricatorInlineCommentInterface::MARKUP_FIELD_BODY);
+    }
+
+    $engine->process();
+
+    $parser->setMarkupEngine($engine);
+
     $spec = $request->getStr('range');
     list($range_s, $range_e, $mask) =
       DifferentialChangesetParser::parseRangeSpecification($spec);