t7400: test failure to add submodule in tracked path

t7400: test failure to add submodule in tracked path

Add a test to ensure failure on adding a submodule to a directory with
tracked contents in the index.

As we are going to refactor and port to C some parts of `git submodule
add`, let's add a test to help ensure no regression is introduced.

Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Based-on-patch-by: Shourya Shukla <periperidip@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>