t: switch $_z40 to $ZERO_OID

t: switch $_z40 to $ZERO_OID

Switch all uses of $_z40 to $ZERO_OID so that they work correctly with
larger hashes.  This commit was created by using the following sed
command to modify all files in the t directory except t/test-lib.sh:

  sed -i 's/\$_z40/$ZERO_OID/g'

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>