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>

t/t3404-rebase-interactive.sh

@@ -555,10 +555,9 @@ test_expect_success 'rebase a detached HEAD' '
 test_expect_success 'rebase a commit violating pre-commit' '
 
 	mkdir -p .git/hooks &&
-	PRE_COMMIT=.git/hooks/pre-commit &&
-	echo "#!/bin/sh" > $PRE_COMMIT &&
-	echo "test -z \"\$(git diff --cached --check)\"" >> $PRE_COMMIT &&
-	chmod a+x $PRE_COMMIT &&
+	write_script .git/hooks/pre-commit <<-\EOF &&
+	test -z "$(git diff --cached --check)"
+	EOF
 	echo "monde! " >> file1 &&
 	test_tick &&
 	test_must_fail git commit -m doesnt-verify file1 &&