Mostly make blame work with DocumentEngine

Summary: Ref T13105. This needs refinement but blame sort of works again, now.

Test Plan: Viewed files in Diffusion and Files; saw blame in Diffusion when viewing in source mode.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19309

src/applications/files/document/PhabricatorDocumentEngine.php

@@ -38,6 +38,10 @@ abstract class PhabricatorDocumentEngine
     return false;
   }
 
+  public function canBlame(PhabricatorDocumentRef $ref) {
+    return false;
+  }
+
   final public function setEncodingConfiguration($config) {
     $this->encodingConfiguration = $config;
     return $this;