t0300: use write_script helper

t0300 creates some helper shell scripts, and marks them with
"!/bin/sh". Even though the scripts are fairly simple, they
can fail on broken shells (specifically, Solaris /bin/sh
will persist a temporary assignment to IFS in a "read"
command).

Rather than work around the problem for Solaris /bin/sh,
using write_script will make sure we point to a known-good
shell that the user has given us.

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

t/t0300-credentials.sh

@@ -14,22 +14,18 @@ test_expect_success 'setup helper scripts' '
 	done
 	EOF
 
-	cat >git-credential-useless <<-\EOF &&
-	#!/bin/sh
+	write_script git-credential-useless <<-\EOF &&
 	. ./dump
 	exit 0
 	EOF
-	chmod +x git-credential-useless &&
 
-	cat >git-credential-verbatim <<-\EOF &&
-	#!/bin/sh
+	write_script git-credential-verbatim <<-\EOF &&
 	user=$1; shift
 	pass=$1; shift
 	. ./dump
 	test -z "$user" || echo username=$user
 	test -z "$pass" || echo password=$pass
 	EOF
-	chmod +x git-credential-verbatim &&
 
 	PATH="$PWD:$PATH"
 '