t7400: add a testcase for submodule status on empty dirs

We have test coverage for "git submodule status" output in various cases, i.e. 1) not-init, not-cloned: status should initially be "missing" 2) init, not-cloned: status should be "missing" 3) not-init, cloned: 4) init, cloned: status should be "up-to-date" after update 4.1) + modified: status should be "modified" after submodule commit 4.2) + modified, committed: status should be "up-to-date" after update but the cases 2) and 3) are not covered. Test that submodule status reports initialized but not cloned submodules as missing to fill the gap in test coverage; this covers case (2) above, but case (3) remains uncovered. Signed-off-by: Peter Kaestle <peter.kaestle@nokia.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>