hooks--update.sample: use hash-agnostic zero OID

hooks--update.sample: use hash-agnostic zero OID

The update sample hook has the zero OID hardcoded as 40 zeros. However,
with the introduction of SHA-256 support, this assumption no longer
holds true. Replace the hardcoded $z40 with a call to

	git hash-object --stdin </dev/null | tr '[0-9a-f]' '0'

so the sample hook becomes hash-agnostic.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>