transport-helper: warn when refspec is not used

transport-helper: warn when refspec is not used

For the modes that need it. In the future we should probably error out,
instead of providing half-assed support.

The reason we want to do this is because if it's not present, the remote
helper might be updating refs/heads/*, or refs/remotes/origin/*,
directly, and in the process fetch will get confused trying to update
refs that are already updated, or older than what they should be. We
shouldn't be messing with the rest of git.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>