Check to see if you have changes in head which have not been pushed before deploying

deploy.rb

namespace :deploy do
  desc "Make sure there is something to deploy"
  task :check_revision, :roles => :web do
    unless `git rev-parse HEAD` == `git rev-parse origin/master`
      puts "WARNING: HEAD is not the same as origin/master"
      puts "Run `git push` to sync changes."
      exit
    end
  end
end
before "deploy", "deploy:check_revision"