Reduce consecutive backslashes to one backslash. Handy when escaped windows paths are passed in.

git-svn-id: https://develop.svn.wordpress.org/trunk@3340 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/functions.php

@@ -2252,6 +2252,7 @@ function register_deactivation_hook($file, $function) {
 }
 
 function plugin_basename($file) {
+	$file = preg_replace('|\\\\+|', '\\\\', $file);
 	$file = preg_replace('/^.*wp-content[\\\\\/]plugins[\\\\\/]/', '', $file);
 	return $file;
 }