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>

commit.c

@@ -331,7 +331,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 	if (get_sha1_hex(bufptr + 5, parent.hash) < 0)
 		return error("bad tree pointer in commit %s",
 			     oid_to_hex(&item->object.oid));
-	item->tree = lookup_tree(parent.hash);
+	item->tree = lookup_tree(&parent);
 	bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
 	pptr = &item->parents;