Improvements to `wp_make_link_relative()`.

* Support relative URL input.
* When the URL being made relative has another URL as a parameter, don't make the second URL relative.

Props voldemortensen.
Fixes #30373.
Built from https://develop.svn.wordpress.org/trunk@30383


git-svn-id: http://core.svn.wordpress.org/trunk@30380 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/formatting.php

@@ -3239,7 +3239,7 @@ function tag_escape($tag_name) {
  * @return string Absolute path.
  */
 function wp_make_link_relative( $link ) {
-	return preg_replace( '|https?://[^/]+(/.*)|i', '$1', $link );
+	return preg_replace( '|^(https?:)?//[^/]+(/.*)|i', '$2', $link );
 }
 
 /**