t7421: introduce a test script for verifying 'summary' output

t7421: introduce a test script for verifying 'summary' output

't7401-submodule-summary.sh' uses 'git add' to add submodules. Therefore,
some commands such as 'git submodule init' and 'git submodule deinit'
do not work as expected.

So, introduce a test script for verifying the 'summary' output for
submodules added using 'git submodule add' and notify regarding the
above mentioned behaviour in t7401 itself.

Mentored-by: Christian Couder <redacted>
Mentored-by: Kaartic Sivaraam <redacted>
Signed-off-by: Shourya Shukla <redacted>
Signed-off-by: Junio C Hamano <redacted>