Make warn_pathless_add() print its warning the first time it is called
and do nothing if called again. This will make it easier to show the
warning on the fly when a relevant condition is detected without
risking showing it multiple times when multiple such conditions hold.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>