Fix the incorrect link target for "Create Revision" as a Menu Item

Summary:
Depends on D20359. Fixes T12098. When you add a new "Form" item and pick "Create Revision", you currently get a bad link. This is because Differential is kind of special and the form isn't usable directly, even though Differential does use EditEngine.

Allow EditEngine to specify a different create URI, then specify the web UI paste-a-diff flow to fix this.

Test Plan:
  - Added "Create Revision" to a portal, clicked it, was sensibly put on the diff flow.
  - Grepped for `getCreateURI()`, the only other real use case is to render the "Create X" dropdowns in the upper right.
    - Clicked one of those, still worked great.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12098

Differential Revision: https://secure.phabricator.com/D20360

src/applications/differential/editor/DifferentialRevisionEditEngine.php

@@ -63,6 +63,10 @@ final class DifferentialRevisionEditEngine
     return $object->getMonogram();
   }
 
+  public function getCreateURI($form_key) {
+    return '/differential/diff/create/';
+  }
+
   protected function getObjectCreateShortText() {
     return pht('Create Revision');
   }

src/applications/transactions/editengine/PhabricatorEditEngine.php

@@ -566,6 +566,18 @@ abstract class PhabricatorEditEngine
     return $this->getObjectViewURI($object);
   }
 
+  /**
+   * @task uri
+   */
+  public function getCreateURI($form_key) {
+    try {
+      $create_uri = $this->getEditURI(null, "form/{$form_key}/");
+    } catch (Exception $ex) {
+      $create_uri = null;
+    }
+
+    return $create_uri;
+  }
 
   /**
    * @task uri

src/applications/transactions/storage/PhabricatorEditEngineConfiguration.php

@@ -227,14 +227,7 @@ final class PhabricatorEditEngineConfiguration
   public function getCreateURI() {
     $form_key = $this->getIdentifier();
     $engine = $this->getEngine();
-
-    try {
-      $create_uri = $engine->getEditURI(null, "form/{$form_key}/");
-    } catch (Exception $ex) {
-      $create_uri = null;
-    }
-
-    return $create_uri;
+    return $engine->getCreateURI($form_key);
   }
 
   public function getIdentifier() {