fixed Diff action

inc/Action/Diff.php

@@ -16,6 +16,7 @@ class Diff extends AbstractAction {
         return AUTH_READ;
     }
 
+    /** @inheritdoc */
     public function preProcess() {
         global $INPUT;
 
@@ -26,4 +27,9 @@ class Diff extends AbstractAction {
         }
     }
 
+    /** @inheritdoc */
+    public function tplContent() {
+        html_diff();
+    }
+
 }