tests: fix existing tests when add an ignore=all submodule

tests: fix existing tests when add an ignore=all submodule

There are tests that rely on "git add <submodule>" to update the in the
reference in the parent repository which have been updated to use the
--force option.

Updated tests:
- t1013-read-tree-submodule.sh ( fixed in: t/lib-submodule-update.sh )
- t2013-checkout-submodule.sh ( fixed in: t/lib-submodule-update.sh )
- t7406-submodule-update.sh
- t7508-status.sh

Signed-off-by: Claus Schneider (Eficode) <claus.schneider@eficode.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>