submodule: fix 'submodule status' when called from a subdirectory

When calling `git submodule status` while in a subdirectory, we are
incorrectly not detecting modified submodules and
thus reporting that all of the submodules are unchanged.

This is because the submodule helper is calling `diff-index` with the
submodule path assuming the path is relative to the current prefix
directory, however the submodule path used is actually relative to the root.

Always pass NULL as the `prefix` when running diff-files on the
submodule, to make sure the submodule's path is interpreted as relative
to the superproject's repository root.

Signed-off-by: Manish Goregaokar <manishsmail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -802,7 +802,8 @@ static void status_submodule(const char *path, const struct object_id *ce_oid,
 			 path, NULL);
 
 	git_config(git_diff_basic_config, NULL);
-	repo_init_revisions(the_repository, &rev, prefix);
+
+	repo_init_revisions(the_repository, &rev, NULL);
 	rev.abbrev = 0;
 	diff_files_args.argc = setup_revisions(diff_files_args.argc,
 					       diff_files_args.argv,

t/t7400-submodule-basic.sh

@@ -356,6 +356,28 @@ test_expect_success 'status should only print one line' '
 	test_line_count = 1 lines
 '
 
+test_expect_success 'status from subdirectory should have the same SHA1' '
+	test_when_finished "rmdir addtest/subdir" &&
+	(
+		cd addtest &&
+		mkdir subdir &&
+		git submodule status >output &&
+		awk "{print \$1}" <output >expect &&
+		cd subdir &&
+		git submodule status >../output &&
+		awk "{print \$1}" <../output >../actual &&
+		test_cmp ../expect ../actual &&
+		git -C ../submod checkout HEAD^ &&
+		git submodule status >../output &&
+		awk "{print \$1}" <../output >../actual2 &&
+		cd .. &&
+		git submodule status >output &&
+		awk "{print \$1}" <output >expect2 &&
+		test_cmp expect2 actual2 &&
+		! test_cmp actual actual2
+	)
+'
+
 test_expect_success 'setup - fetch commit name from submodule' '
 	rev1=$(cd .subrepo && git rev-parse HEAD) &&
 	printf "rev1: %s\n" "$rev1" &&