Convert lookup_tree to struct object_id

Convert the lookup_tree function to take a pointer to struct object_id.

The commit was created with manual changes to tree.c, tree.h, and
object.c, plus the following semantic patch:

@@
@@
- lookup_tree(EMPTY_TREE_SHA1_BIN)
+ lookup_tree(&empty_tree_oid)

@@
expression E1;
@@
- lookup_tree(E1.hash)
+ lookup_tree(&E1)

@@
expression E1;
@@
- lookup_tree(E1->hash)
+ lookup_tree(E1)

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

object.c

@@ -197,7 +197,7 @@ struct object *parse_object_buffer(const unsigned char *sha1, enum object_type t
 			obj = &blob->object;
 		}
 	} else if (type == OBJ_TREE) {
-		struct tree *tree = lookup_tree(sha1);
+		struct tree *tree = lookup_tree(&oid);
 		if (tree) {
 			obj = &tree->object;
 			if (!tree->buffer)