t5407: fix test to cover intended arguments

Test 8 in t5407 appears to be an accidental exact duplicate of of test 5;
the testcode is identical and has identical repo state, but the test
description is different and suggests that rebase -m followed by rebase
--skip was what was actually supposed to be tested.  Modify the test to
include the -m option.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5407-post-rewrite-hook.sh

@@ -113,7 +113,7 @@ test_expect_success 'git rebase -m' '
 test_expect_success 'git rebase -m --skip' '
 	git reset --hard D &&
 	clear_hook_input &&
-	test_must_fail git rebase --onto A B &&
+	test_must_fail git rebase -m --onto A B &&
 	test_must_fail git rebase --skip &&
 	echo D > foo &&
 	git add foo &&