The test_must_fail function should only be used for git commands since
we should assume that external commands work sanely. Since apply_patch
wraps a sed and git invocation, rewrite it to accept an `!` argument
which would cause only the git command to be prefixed with
`test_must_fail`.
Signed-off-by: Denton Liu <liu.denton@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>