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>

t/t7400-submodule-basic.sh

@@ -196,6 +196,17 @@ test_expect_success 'submodule add to .gitignored path with --force' '
 	)
 '
 
+test_expect_success 'submodule add to path with tracked content fails' '
+	(
+		cd addtest &&
+		echo "'\''dir-tracked'\'' already exists in the index" >expect &&
+		mkdir dir-tracked &&
+		test_commit foo dir-tracked/bar &&
+		test_must_fail git submodule add "$submodurl" dir-tracked >actual 2>&1 &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'submodule add to reconfigure existing submodule with --force' '
 	(
 		cd addtest-ignore &&