Issue #1989996 by swentel: Fixed @config_file() placeholder not replaced in popup.

core/modules/config/lib/Drupal/config/Controller/ConfigController.php

@@ -94,7 +94,7 @@ public function diff($config_file) {
 
     // @todo Remove use of drupal_set_title() when
     //   http://drupal.org/node/1871596 is in.
-    drupal_set_title('View changes of @config_file', array('@config_file' => $config_file));
+    drupal_set_title(t('View changes of @config_file', array('@config_file' => $config_file)), PASS_THROUGH);
 
     return $output;
   }

core/modules/config/lib/Drupal/config/Tests/ConfigImportUITest.php

@@ -151,6 +151,7 @@ function testImportDiff() {
 
     // Load the diff UI and verify that the diff reflects the change.
     $this->drupalGet('admin/config/development/sync/diff/' . $config_name);
+    $this->assertTitle(format_string('View changes of @config_name | Drupal', array('@config_name' => $config_name)));
 
     // Reset data back to original, and remove a key
     $staging_data = $original_data;