t5406: use write_script() instead of birthing shell script manually

t5406: use write_script() instead of birthing shell script manually

Take advantage of write_script() to abstract-away details of shell
script creation, thus allowing the reader to focus on script content.
Readability benefits, particularly in this case, since the script body
was buried in a noisy one-liner subshell responsible for emitting
boilerplate and body.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>