stash: add t3906 for submodule updates

Test that the stash apply command updates the work tree as expected for
changes which don't result in conflicts no matter if submodule changes
are staged or not.

Add the KNOWN_FAILURE_STASH_DOES_IGNORE_SUBMODULE_CHANGES and reuse two
other already present switches to expect the known failure that stash
does ignore submodule changes.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! checkout-index: add t2025 for submodule updates

TODO: fix or expect failing tests

Test that the checkout-index command updates the work tree as expected
(for submodule changes which don't result in conflicts) with or without
the '-f' flag.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! reminder of what is still left to do ...