Introduce `'the_category_list'` filter.

Used to filter categories as structured data, before building markup in
`get_the_category_list()`.

We use this filter in addition to upstream filters (such as
'get_the_categories'`) because those upstream filters are used in numerous
contexts, while `'the_category_list'` is always used for generating markup
for display.

Props KevinB, ericlewis, SergeyBiryukov, DrewAPicture.
Fixes #9227.
Built from https://develop.svn.wordpress.org/trunk@34625


git-svn-id: http://core.svn.wordpress.org/trunk@34589 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/category-template.php

@@ -175,7 +175,17 @@ function get_the_category_list( $separator = '', $parents='', $post_id = false )
 		return apply_filters( 'the_category', '', $separator, $parents );
 	}
 
-	$categories = get_the_category( $post_id );
+	/**
+	 * Filter the categories before building the category list.
+	 *
+	 * @since 4.4.0
+	 *
+	 * @param array    $categories An array of the post's categories.
+	 * @param int|bool $post_id    ID of the post we're retrieving categories for. When `false`, we assume the
+	 *                             current post in the loop.
+	 */
+	$categories = apply_filters( 'the_category_list', get_the_category( $post_id ), $post_id );
+
 	if ( empty( $categories ) ) {
 		/** This filter is documented in wp-includes/category-template.php */
 		return apply_filters( 'the_category', __( 'Uncategorized' ), $separator, $parents );

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-34624';
+$wp_version = '4.4-alpha-34625';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.