Sitemaps: Pass full paths to home_url()

[Chouby]

When using plain permalinks, WP_Sitemaps_Index::get_index_url() builds the sitemap index url like this:
add_query_arg( 'sitemap', 'index', home_url( '/' ) );

See: ​https://github.com/WordPress/WordPress/blob/fe823d698f5e57135c102e02d7e23d4019ac59d5/wp-includes/sitemaps/class-wp-sitemaps-index.php#L78

There is no issue on a fresh install but it causes issues with plugins or themes using the filter home_url. Any function hooked to this filter will interpret that the url being built is the homepage url when it's actually the sitemap index url which is being built.

I propose to change this to:
home_url( '/?sitemap=index' );
The result should be the same for WordPress but would be less confusing for users of the home_url filter.

I noticed 2 other similar constructions in: ​https://github.com/WordPress/WordPress/blob/fe823d698f5e57135c102e02d7e23d4019ac59d5/wp-includes/sitemaps/class-wp-sitemaps-renderer.php#L70 and ​https://github.com/WordPress/WordPress/blob/fe823d698f5e57135c102e02d7e23d4019ac59d5/wp-includes/sitemaps/class-wp-sitemaps-renderer.php#L101

[Chouby]

For information, I looked at other places of WordPress how is handled this type of case, and I found:
$permalink = home_url( '?p=' . $post->ID ); in get_permalink().
$link = home_url( '?page_id=' . $post->ID ); in _get_page_link().

[Chouby]

50592.2.diff​ fixes an additional case that I had first overlooked.

[swissspidy]

In 48470:

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.