Code Modernization: Use dirname() with the $levels parameter.

PHP 7.0 introduced the `$levels` parameter to the `dirname()` function, which means nested calls to `dirname()` are no longer needed.

Note: This is enforced by WPCS 3.0.0.

Reference: [https://www.php.net/manual/en/function.dirname.php PHP Manual: dirname()].

Follow-up to [56141].

Props jrf.
See #59161, #58831.

git-svn-id: https://develop.svn.wordpress.org/trunk@56552 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/theme.php

@@ -156,7 +156,7 @@ function get_page_templates( $post = null, $post_type = 'page' ) {
  * @return string
  */
 function _get_template_edit_filename( $fullpath, $containingfolder ) {
-	return str_replace( dirname( dirname( $containingfolder ) ), '', $fullpath );
+	return str_replace( dirname( $containingfolder, 2 ), '', $fullpath );
 }
 
 /**

src/wp-admin/maint/repair.php

@@ -7,7 +7,7 @@
  */
 define( 'WP_REPAIRING', true );
 
-require_once dirname( dirname( __DIR__ ) ) . '/wp-load.php';
+require_once dirname( __DIR__, 2 ) . '/wp-load.php';
 
 header( 'Content-Type: text/html; charset=utf-8' );
 ?>