Another fix for the duplicate task-arguments problem.

task.rb

    # Invoke all the prerequisites of a task in parallel.
    def invoke_prerequisites_concurrently(task_args, invocation_chain) # :nodoc:
      futures = prerequisite_tasks.collect do |p|
        prereq_args = task_args.new_scope(p.arg_names)
#         application.thread_pool.future(p) do |r|
#           r.invoke_with_call_chain(prereq_args, invocation_chain)
#         end
        application.thread_pool.future do
          p.invoke_with_call_chain(prereq_args, invocation_chain)
        end
      end
      futures.each { |f| f.value }
    end