git:warn_unpushed_changes cap task

git.rb

Capistrano::Configuration.instance(:must_exist).load do
  namespace :git do
    task :warn_unpushed_changes do
      master_rev = `git rev-parse master`.strip
      origin_master_rev = `git rev-parse origin/master`.strip

      if master_rev != origin_master_rev
        puts "*" * 80
        answer = Capistrano::CLI.ui.ask(" You have local commits that have yet to be pushed to origin.  Continue? (y/N)")
        exit(1) unless answer.downcase == 'y'
      end
    end
  end

  before "deploy", "git:warn_unpushed_changes"
  before "deploy:migrations", "git:warn_unpushed_changes"
end