I found somewhere else that the lines to edit in wp-includes/comment-template.php are as follows…
function comments_popup_link( $zero = false, $one = false, $more = false, $css_class = '', $none = false, $nofollow = true )
$title = esc_attr( get_the_title() ); if($nofollow) { $nofollow = ' rel="nofollow"';}
echo apply_filters( 'comments_popup_link_attributes', '' );
Hope this helps