diff: add test for --no-index executed outside repo

470faf9 diff: move no-index detection to builtin/diff.c breaks the error
message for "git diff --no-index", when the command is executed outside
of a git repository and the wrong number of arguments are given. 6df5762
diff: don't read index when --no-index is given fixes the problem.

Add a test to guard against similar breakages in the future.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4053-diff-no-index.sh

@@ -44,4 +44,15 @@ test_expect_success 'git diff outside repo with broken index' '
 	)
 '
 
+test_expect_success 'git diff --no-index executed outside repo gives correct error message' '
+	(
+		GIT_CEILING_DIRECTORIES=$TRASH_DIRECTORY/non &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git diff --no-index a 2>actual.err &&
+		echo "usage: git diff --no-index <path> <path>" >expect.err &&
+		test_cmp expect.err actual.err
+	)
+'
+
 test_done