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>

fsck.c

@@ -358,7 +358,7 @@ static int fsck_walk_tree(struct tree *tree, void *data, struct fsck_options *op
 			continue;
 
 		if (S_ISDIR(entry.mode)) {
-			obj = &lookup_tree(entry.oid->hash)->object;
+			obj = &lookup_tree(entry.oid)->object;
 			if (name)
 				put_object_name(options, obj, "%s%s/", name,
 					entry.path);