A git ref in every response!

application_controller.rb

class ApplicationController < ActionController::Base
  before_filter {
    headers['X-Refspec'] = TC::Services.refspec
  }
end

services.rb

module TC
  class Services
    # The git ref that's currently running. Using ||= in a class method
    # means we only shell out once, when the instance is booted.
    def self.refspec
      @_refspec ||= `git rev-parse --short HEAD 2>/dev/null`.strip
    end

    # ...
  end
end