MDL-35628 performance: Remove dirname() where possible.

dirname() is a slow function compared with __DIR__ and using
'/../'.  Moodle has a large number of legacy files that are included
each time a page loads and is not able to use an autoloader as it is
functional code.  This allows those required includes to perform as
best as possible in this situation.

admin/cli/backup.php

@@ -25,7 +25,7 @@
 
 define('CLI_SCRIPT', 1);
 
-require(dirname(dirname(dirname(__FILE__))).'/config.php');
+require(__DIR__.'/../../config.php');
 require_once($CFG->libdir.'/clilib.php');
 require_once($CFG->dirroot . '/backup/util/includes/backup_includes.php');