Refactor submodule push check to use string list instead of integer

This allows us to tell the user which submodules have not been pushed.
Additionally this is helpful when we want to automatically try to push
submodules that have not been pushed.

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