tree-walk: convert tree entry functions to object_id

Convert get_tree_entry and find_tree_entry to take pointers to struct
object_id.

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

archive.c

@@ -397,8 +397,8 @@ static void parse_treeish_arg(const char **argv,
 		unsigned int mode;
 		int err;
 
-		err = get_tree_entry(tree->object.oid.hash, prefix,
-				     tree_oid.hash, &mode);
+		err = get_tree_entry(&tree->object.oid, prefix, &tree_oid,
+				     &mode);
 		if (err || !S_ISDIR(mode))
 			die("current working directory is untracked");