capistrano custom multistage task

multistage.rb

# NB! must be loaded after [require 'capistrano/ext/multistage']
namespace :multistage do
  desc "List available stages"
  task :list, :except => { :no_release => true,:stages => true } do
    print "#{stages.join(',')}\n"
  end
end

# overload config to allow stage listing w/o selected stage
if callbacks[:start]
  idx = callbacks[:start].index { |callback| callback.source == 'multistage:ensure' }
  callbacks[:start].delete_at(idx)
  on :start, 'multistage:ensure', :except => stages + ['multistage:prepare', 'multistage:list']
end