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