t5562: use $ZERO_OID

This test uses $_z40 to express an all-zeros object ID, which doesn't
work for SHA-256.  Use $ZERO_OID instead, which is the right size for
all hash values.

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

t/t5562-http-backend-content-length.sh

@@ -59,7 +59,7 @@ test_expect_success 'setup' '
 	printf done | packetize >>fetch_body &&
 	test_copy_bytes 10 <fetch_body >fetch_body.trunc &&
 	hash_next=$(git commit-tree -p HEAD -m next HEAD^{tree}) &&
-	printf "%s %s refs/heads/newbranch\\0report-status\\n" "$_z40" "$hash_next" | packetize >push_body &&
+	printf "%s %s refs/heads/newbranch\\0report-status\\n" "$ZERO_OID" "$hash_next" | packetize >push_body &&
 	printf 0000 >>push_body &&
 	echo "$hash_next" | git pack-objects --stdout >>push_body &&
 	test_copy_bytes 10 <push_body >push_body.trunc &&