t2107: mark passing test as success

This failed on the branch where it was introduced, but was fixed
by merging with 6e67619 (Merge branch 'jn/parse-options-extra').

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2107-update-index-basic.sh

@@ -13,7 +13,7 @@ test_expect_success 'update-index --nonsense fails' '
 	test -s msg
 '
 
-test_expect_failure 'update-index --nonsense dumps usage' '
+test_expect_success 'update-index --nonsense dumps usage' '
 	test_expect_code 129 git update-index --nonsense 2>err &&
 	grep "[Uu]sage: git update-index" err
 '