t3404: use write_script

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>