Propagate the "ContextObject" to Remarkup rendering in timelines

Summary:
Ref T13602. Currently, timeline comment rendering does not (by default) propagate the context object to the rendering layer.

This means that `@mentions` of users who can't see the object aren't rendered properly (currently: they show up as blue, but should show up as grey).

Pass the context down the stack and into the remarkup engine.

Test Plan: {F8382905}

Maniphest Tasks: T13602

Differential Revision: https://secure.phabricator.com/D21548

src/applications/transactions/engine/PhabricatorTimelineEngine.php

@@ -84,6 +84,7 @@ abstract class PhabricatorTimelineEngine
 
     return $view
       ->setViewer($viewer)
+      ->setObject($object)
       ->setObjectPHID($object->getPHID())
       ->setTransactions($xactions);
   }

src/applications/transactions/view/PhabricatorApplicationTransactionView.php

@@ -9,6 +9,7 @@ class PhabricatorApplicationTransactionView extends AphrontView {
   private $engine;
   private $showEditActions = true;
   private $isPreview;
+  private $object;
   private $objectPHID;
   private $shouldTerminate = false;
   private $quoteTargetID;
@@ -41,6 +42,16 @@ class PhabricatorApplicationTransactionView extends AphrontView {
     return $this->quoteTargetID;
   }
 
+  public function setObject(
+    PhabricatorApplicationTransactionInterface $object) {
+    $this->object = $object;
+    return $this;
+  }
+
+  private function getObject() {
+    return $this->object;
+  }
+
   public function setObjectPHID($object_phid) {
     $this->objectPHID = $object_phid;
     return $this;
@@ -238,6 +249,12 @@ class PhabricatorApplicationTransactionView extends AphrontView {
 
       $engine = id(new PhabricatorMarkupEngine())
         ->setViewer($this->getViewer());
+
+      $object = $this->getObject();
+      if ($object) {
+        $engine->setContextObject($object);
+      }
+
       foreach ($this->transactions as $xaction) {
         if (!$xaction->hasComment()) {
           continue;