t7400: add a testcase for submodule status on empty dirs

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>