t4000: make hash size independent

Use $ZERO_OID instead of hard-coding a fixed size all-zeros object ID.

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

t/t4000-diff-format.sh

@@ -78,7 +78,7 @@ test_expect_success 'git diff-files --no-patch --patch shows the patch' '
 
 test_expect_success 'git diff-files --no-patch --patch-with-raw shows the patch and raw data' '
 	git diff-files --no-patch --patch-with-raw >actual &&
-	grep -q "^:100644 100755 .* 0000000000000000000000000000000000000000 M	path0\$" actual &&
+	grep -q "^:100644 100755 .* $ZERO_OID M	path0\$" actual &&
 	tail -n +4 actual >actual-patch &&
 	compare_diff_patch expected actual-patch
 '