oid_pos(): access table through const pointers

When we are looking up an oid in an array, we obviously don't need to
write to the array. Let's mark it as const in the function interfaces,
as well as in the local variables we use to derference the void pointer
(note a few cases use pointers-to-pointers, so we mark everything
const).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

oid-array.c

@@ -22,9 +22,9 @@ void oid_array_sort(struct oid_array *array)
 	array->sorted = 1;
 }
 
-static const struct object_id *oid_access(size_t index, void *table)
+static const struct object_id *oid_access(size_t index, const void *table)
 {
-	struct object_id *array = table;
+	const struct object_id *array = table;
 	return &array[index];
 }