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>

hash-lookup.h

@@ -1,10 +1,10 @@
 #ifndef HASH_LOOKUP_H
 #define HASH_LOOKUP_H
 
-typedef const struct object_id *oid_access_fn(size_t index, void *table);
+typedef const struct object_id *oid_access_fn(size_t index, const void *table);
 
 int oid_pos(const struct object_id *oid,
-	    void *table,
+	    const void *table,
 	    size_t nr,
 	    oid_access_fn fn);