Search ContentFilter onlyFirstComment() does not work correctly

The code's intention is that you can create a ContentFilter instance with onlyFirstComment() set to TRUE to group and only return the first result in a content item that requires comments (i.e. topics). At a technical level, this is implemented like so (for MySQL):

			/* Only first comment? */
			if ( $filter->onlyFirstComment )
			{
				$clause[] = "index_title IS NOT NULL";
			}

The problem is, in practice this does not work as expected because the topic title is set for "index_title" for every single post within the topic. Subsequently, if I create a ContentFilter and set the onlyFirstComment property to TRUE, all comments within the topic are still returned.

I rebuilt the search index to verify the search index behavior wasn't legacy or something.

Note that my testing around this was limited explicitly to topics. I do not know if this is a topics-specific concern, or applies to everything with the $firstCommentRequired property set.

Sample code to show the issue:

		$search = Query::init();
		$search->setLimit(10);
		$search->setPage(1);
		$search->setOrder( Query::ORDER_NEWEST_UPDATED );

		$contentFilters[] = ContentFilter::init( 'IPS\forums\Topic', TRUE, TRUE, FALSE )->onlyFirstComment();

		$search->filterByContent( $contentFilters );

		$results = $search->search( NULL, NULL);

With the onlyFirstComment() flag we should only be seeing the first comment in the topics, but all replies will be returned.