deploy.rb

namespace :deploy do
  desc "Make sure there are no changes that need to be pushed."
  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"