disable complete ignorance of submodules for index <-> HEAD diff

If the value of ignore for submodules is set to "all" we would not show
whats actually committed during status or diff. This can result in the
user committing unexpected submodule references. Lets be nicer and always
show whats in the index.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>