t3404: use write_script

The test uses hardcoded #!/bin/sh to create a pre-commit hook script. Because the generated script uses $(command substitution), which is not supported by /bin/sh on some platforms (e.g. Solaris), the resulting pre-commit always fails. Which is not noticeable as the test that uses the hook is about checking the behaviour of the command when the hook fails ;-), but nevertheless it is not testing what we wanted to test. Use write_script so that the resulting script is run under the same shell our scripted Porcelain commands are run, which must support the necessary $(construct). Helped-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>