Explicitly declare hierarchical=false for post_tag in create_initial_taxonomies().

In certain edge cases, such as during the automated test suite,
create_initial_taxonomies() can be called in a situation where a non-default
permastruct setting has been saved outside of wp-admin. This causes the
'hierarchical' setting for the post_tag taxonomy to remain unset, leading to
PHP notices in get_term_link().

See #29827.

git-svn-id: https://develop.svn.wordpress.org/trunk@29859 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -41,6 +41,7 @@ function create_initial_taxonomies() {
 				'ep_mask' => EP_CATEGORIES,
 			),
 			'post_tag' => array(
+				'hierarchical' => false,
 				'slug' => get_option('tag_base') ? get_option('tag_base') : 'tag',
 				'with_front' => ! get_option('tag_base') || $wp_rewrite->using_index_permalinks(),
 				'ep_mask' => EP_TAGS,