merge hook tests: use 'test_must_fail' instead of '!'

Signed-off-by: Benoit Pierre <benoit.pierre@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7505-prepare-commit-msg-hook.sh

@@ -154,7 +154,7 @@ test_expect_success 'with failing hook' '
 	head=`git rev-parse HEAD` &&
 	echo "more" >> file &&
 	git add file &&
-	! GIT_EDITOR="\"\$FAKE_EDITOR\"" git commit -c $head
+	test_must_fail env GIT_EDITOR="\"\$FAKE_EDITOR\"" git commit -c $head
 
 '
 
@@ -163,7 +163,7 @@ test_expect_success 'with failing hook (--no-verify)' '
 	head=`git rev-parse HEAD` &&
 	echo "more" >> file &&
 	git add file &&
-	! GIT_EDITOR="\"\$FAKE_EDITOR\"" git commit --no-verify -c $head
+	test_must_fail env GIT_EDITOR="\"\$FAKE_EDITOR\"" git commit --no-verify -c $head
 
 '