t4045: make hash-size independent

Replace a hard-coded all-zeros object ID with a use of $ZERO_OID.

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

t/t4045-diff-relative.sh

@@ -70,7 +70,7 @@ check_raw () {
 	expect=$1
 	shift
 	cat >expected <<-EOF
-	:000000 100644 0000000000000000000000000000000000000000 $blob A	$expect
+	:000000 100644 $ZERO_OID $blob A	$expect
 	EOF
 	test_expect_success "--raw $*" "
 		git -C '$dir' diff --no-abbrev --raw $* HEAD^ >actual &&