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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30383 602fd350-edb4-49c9-b593-d223f7449a82