Sitemaps: Pass full paths to `home_url()` calls.

This makes it easier for plugins using the `home_url` filter to detect sitemap URLs.

Props Chouby.
Fixes #50592.
Built from https://develop.svn.wordpress.org/trunk@48470


git-svn-id: https://core.svn.wordpress.org/trunk@48239 1a063a9b-81f0-0310-95a4-ce76da25c4cd