t5562: replace /dev/zero with a pipe from generate_zero_bytes

To help platforms that lack /dev/zero (e.g. NonStop), replace use
of /dev/zero to feed "git http-backend" with a pipe of output from
the generate_zero_bytes helper.

Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>