assets.rb

after "deploy:finalize_update", "deploy:assets:conditionally_precompile"

namespace :deploy do
  namespace :assets do
        
    desc "Remove callback for asset precompiling unless assets were updated based on Git fetch/merge."
    task :conditionally_precompile, :roles => assets_role, :except => { :no_release => true } do
      updated_assets = capture("cd #{latest_release}; git diff --name-only #{old_commit} #{new_commit}", :except => { :no_release => true }).split.find_all{|fname| File.dirname(fname).match(/^(vendor|app|lib)\/assets/) }
      
      if(updated_assets.empty?)
        callback = callbacks[:after].find{|c| c.source == "deploy:assets:precompile" }
        callbacks[:after].delete(callback)
        logger.info "Skipping asset pre-compilation because there were no asset changes"
      else
        logger.info("#{updated_assets.length} updated assets. Will precompile.")
      end
    end
    
  end
end

git.rb

before "deploy:update_code", "deploy:git:set_old_commit"
before "deploy:finalize_update", "deploy:git:set_new_commit"

set :git_roles, [:app]

namespace :deploy do
  namespace :git do
    
    def which_commit(rev="HEAD")
      capture("cd #{current_path}; git rev-parse #{rev}", :except => { :no_release => true }).chomp.strip
    end
    
    desc "Set variable for commit before update"
    task :set_old_commit, :roles => git_roles, :except => { :no_release => true } do
      set :old_commit, which_commit
    end
    
    desc "Set variable for commit after update"
    task :set_new_commit, :roles => git_roles, :except => { :no_release => true} do
      set :new_commit, which_commit
    end
    
    desc "Update code via git-pull"
    task :pull, :roles => git_roles, :except => { :no_release => true} do
      run "cd #{current_path}; git pull -q"
    end
    
    desc "Update code via git-fetch and reset"
    task :fetch, :roles => git_roles, :except => { :no_release => true} do
      run "cd #{current_path}; git fetch origin; git reset --hard #{branch}"
    end
  end
end