object: convert lookup_object() to use object_id

There are no callers left of lookup_object() that aren't just passing us
the "hash" member of a "struct object_id". Let's take the whole struct,
which gets us closer to removing all raw sha1 variables.  It also
matches the existing conversions of lookup_blob(), etc.

The conversions of callers were done by hand, but they're all mechanical
one-liners.

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

commit.c

@@ -57,7 +57,7 @@ struct commit *lookup_commit_or_die(const struct object_id *oid, const char *ref
 
 struct commit *lookup_commit(struct repository *r, const struct object_id *oid)
 {
-	struct object *obj = lookup_object(r, oid->hash);
+	struct object *obj = lookup_object(r, oid);
 	if (!obj)
 		return create_object(r, oid->hash,
 				     alloc_commit_node(r));