current_app.rb

def extract_app_in_dir(dir)
  return unless remotes = git_remotes(dir)
  current_app = false

  begin
    current_app = File.read(FileUtils.pwd + '/.current-heroku-app').chomp
  rescue Errno::ENOENT
  end

  if remote = options[:remote]
    remotes[remote]
  elsif remote = extract_app_from_git_config
    remotes[remote]
  else
    apps = remotes.values.uniq

    if apps.size == 1
      apps.first
    elsif current_app && apps.include?(current_app)
      current_app
    else
      raise(Heroku::Command::CommandFailed, "Multiple apps in folder and no app specified.\nSpecify app with --app APP.") unless options[:ignore_no_app]
    end
  end
end