lib-submodule-update.sh: do not use ./. as submodule remote

lib-submodule-update.sh: do not use ./. as submodule remote

Adding the repository itself as a submodule does not make sense in the
real world. In our test suite we used to do that out of convenience in
some tests as the current repository has easiest access for setting up
'just a submodule'.

However this doesn't quite test the real world, so let's do not follow
this pattern any further and actually create an independent repository
that we can use as a submodule.

When using './.' as the remote the superproject and submodule share the
same objects, such that testing if a given sha1 is a valid commit works
in either repository.  As running commands in an unpopulated submodule
fall back to the superproject, this happens in `reset_work_tree_to`
to determine if we need to populate the submodule. Fix this bug by
checking in the actual remote now.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>