t7508: use $ZERO_OID instead of hard-coded constant

Use the ZERO_OID variable to abbreviate the all-zeros object ID for
maintainability and to avoid depending on a specific size for the hash.

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

t/t7508-status.sh

@@ -837,7 +837,7 @@ EOF
 '
 
 cat >expect <<EOF
-:100644 100644 $EMPTY_BLOB 0000000000000000000000000000000000000000 M	dir1/modified
+:100644 100644 $EMPTY_BLOB $ZERO_OID M	dir1/modified
 EOF
 test_expect_success 'status refreshes the index' '
 	touch dir2/added &&