Add a second testcase for handling invalid strategies in git-merge

This one tests '-s index' which is interesting because git-merge-index
is an existing git command but it is not a valid strategy.
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7600-merge.sh

@@ -230,6 +230,10 @@ test_expect_success 'test option parsing' '
 	test_must_fail git merge
 '
 
+test_expect_success 'reject non-strategy with a git-merge-foo name' '
+	test_must_fail git merge -s index c1
+'
+
 test_expect_success 'merge c0 with c1' '
 	git reset --hard c0 &&
 	git merge c1 &&