Speed up rake assets:precompile *without* any sprockets or git hacks.

deploy.rb

  desc "precompile assets (only if needed)"
  task :precompile_assets do
    puts "Diffing assets to check if we need to precompile"
    precompile = true

    # Only do this if we have a previous release
    if previous_release && latest_release
      puts "Comparing asset sources of #{previous_release} and #{latest_release}."

      # Gather all the names of the source files from your local machine.
      # NOTE: it would be better to diff the filename lists of the previous and lastest release, but this will function.
      files = ['Gemfile.lock', 'config/routes.rb']
      ['app/assets', 'lib/assets', 'vendor/assets'].map{|d| Dir["#{d}/**/*.*"].each{ |f| files << f }}

      begin
        diff_commands = []
        files.each do |file|
          diff_commands << "diff -w -q '#{previous_release}/#{file}' '#{latest_release}/#{file}'"
        end

        # Diff & copy, raises CommandError if any of the diffs fail
        run diff_commands.join(' && ')
        run "cp -R -p #{previous_release}/public/assets/ #{latest_release}/public/assets/"
        precompile = false
      rescue Capistrano::CommandError
        puts "Diff detected, so we need to recompile our assets"
        precompile = true
      end
    end

    # Do a normal precompile if anything broke or on a force precompile
    precompile = true if ENV['FORCE_PRECOMPILE']
    run "cd #{latest_release} && bundle exec rake i18n:js:export assets:precompile RAILS_ENV=production" if precompile
  end