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>

t/t4002-diff-basic.sh

@@ -131,7 +131,7 @@ cmp_diff_files_output () {
     # object ID for the changed files because it wants you to look at the
     # filesystem.
     sed <"$2" >.test-tmp \
-	-e '/^:000000 /d;s/'$_x40'\( [MCRNDU][0-9]*\)	/'$_z40'\1	/' &&
+	-e '/^:000000 /d;s/'$_x40'\( [MCRNDU][0-9]*\)	/'$ZERO_OID'\1	/' &&
     test_cmp "$1" .test-tmp
 }