In particular, when moving back to a commit without a given submodule
and then moving back forward to a commit with the given submodule,
we shouldn't complain that updating would lose untracked file in
the submodule, because git currently does not checkout subprojects
during superproject check-out.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>