Prevent extra db queries in WP_Comment::get_children().

`WP_Comment_Query::fill_descendants()` queries for a comment tree in a way that
minimizes database overhead, and places the located descendants with their
proper parents. However, it doesn't touch leaf nodes - comments with no
children - so future calls to `get_children()` on those comment objects
result in unnecessary database queries. To prevent this, `fill_descendants()`
now sets a `populated_children` flag on all located `WP_Comment` objects.

See #8071.

git-svn-id: https://develop.svn.wordpress.org/trunk@34730 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-comment-query.php

@@ -922,6 +922,11 @@ class WP_Comment_Query {
 				}
 			}
 
+			// Set the 'populated_children' flag, to ensure additional database queries aren't run.
+			foreach ( $ref as $_ref ) {
+				$_ref->populated_children( true );
+			}
+
 			$comments = $threaded_comments;
 		} else {
 			$comments = $all_comments;