deploy.rb

namespace :git do
  task :check_heads do
    my_head = run_locally( 'git ls-remote . refs/heads/master' )
    origin_head = run_locally( 'git ls-remote origin refs/heads/master' )
    if my_head != origin_head
      raise "Your master is not in sync with origin"
    end
  end
end

before 'deploy:update_code', 'git:check_heads'