gistfile1.js

var root = io.File("~/hoge");
var target = io.File("~/var");
var path="";

while (root.parent){
        print(root.path);
        if (target.path.indexOf(root.path) == 0){
                break;
        }
        root = root.parent;
        path += "../";
}

path += target.path.substring(root.path.length + 1);

print("=====");
print(path);