submodule_needs_pushing(): explain the behaviour when we cannot answer

submodule_needs_pushing(): explain the behaviour when we cannot answer

When we do not have commits that are involved in the update of the
superproject in our copy of submodule, we cannot tell if the remote
end needs to acquire these commits to be able to check out the
superproject tree.  Explain why we answer "no there is no need/point
in pushing from our submodule repository" in this case.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>