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>
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index e4f4c4d..1e81b33 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -426,7 +426,7 @@
 	git commit -m "commit" &&
 	{
 		git ls-tree HEAD &&
-		printf "160000 commit $_z40\\tbroken\\n"
+		printf "160000 commit $ZERO_OID\\tbroken\\n"
 	} >broken-tree &&
 	echo "add broken entry" >msg &&