Allow `get_comments_number()` to officially accept `$post` or `$post_id`. Adds unit tests.

Props coffee2code, JanHenkG.
Fixes #26240.

Built from https://develop.svn.wordpress.org/trunk@28558


git-svn-id: http://core.svn.wordpress.org/trunk@28384 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/comment-template.php

@@ -682,16 +682,13 @@ function comments_link( $deprecated = '', $deprecated_2 = '' ) {
  * @return int The number of comments a post has.
  */
 function get_comments_number( $post_id = 0 ) {
-	$post_id = absint( $post_id );
+	$post = get_post( $post_id );
 
-	if ( !$post_id )
-		$post_id = get_the_ID();
-
-	$post = get_post($post_id);
-	if ( ! isset($post->comment_count) )
+	if ( ! isset( $post->comment_count ) ) {
 		$count = 0;
-	else
+	} else {
 		$count = $post->comment_count;
+	}
 
 	/**
 	 * Filter the returned comment count for a post.