submodule: teach unpack_trees() to remove submodule contents

Implement the functionality needed to enable work tree manipulating
commands to that a deleted submodule should not only affect the index
(leaving all the files of the submodule in the work tree) but also to
remove the work tree of the superproject (including any untracked
files).

That will only work properly when the submodule uses a gitfile instead of
a .git directory and no untracked files are present. Otherwise the removal
will fail with a warning (which is just what happened until now).

Extend rmdir_or_warn() to remove the directories of those submodules which
are scheduled for removal. Also teach verify_clean_submodule() to check
that a submodule configured to be removed is not modified before scheduling
it for removal.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
submodule: teach unpack_trees() to repopulate submodules

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
submodule: teach unpack_trees() to update submodules

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! Teach checkout to recursively checkout submodules

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>