add a test for git-send-email for non-threaded mails

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9001-send-email.sh

@@ -621,4 +621,14 @@ test_expect_success 'in-reply-to but no threading' '
 	grep "In-Reply-To: <in-reply-id@example.com>"
 '
 
+test_expect_failure 'no in-reply-to and no threading' '
+	git send-email \
+		--dry-run \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--nothread \
+		$patches $patches >stdout &&
+	! grep "In-Reply-To: " stdout
+'
+
 test_done