git submodule add: Require the new --force option to add ignored paths

git submodule add: Require the new --force option to add ignored paths

To make the behavior of "git submodule add" more consistent with "git add"
ignored submodule paths should not be silently added when they match an
entry in a .gitignore file. To be able to override that default behavior
in the same way as we can do that for "git add", the new option "--force"
is introduced.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>