t7400: testcase for submodule status on unregistered inner git repos

t7400: testcase for submodule status on unregistered inner git repos

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: status should ignore the inner git-repo
  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

the case 3) is not covered yet.

Test that submodule status reports an inner git repo as unknown, while
it is not added to the superproject.  This covers case (3).

Signed-off-by: Peter Kaestle <redacted>
Signed-off-by: Junio C Hamano <redacted>