I'll trim them down. Here are the ones that I suggest keeping:

* `Gemfile.lock` - the rewrite sets up a mini-rails-app as dev environment
* `pkg` - `rake release` will generate temporary packaged gem files here when releasing new versions
* `tmp` - sprockets in the mini-rails-app will use this for caching
* `log` - likewise the mini-rails-app will generate logs here

How does that sound to you?