t0300: use write_script helper

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>