Net::FTP#chdir

# File tmp/rubies/ruby-3.0.5/lib/net/ftp.rb, line 1170
def chdir(dirname)
  if dirname == ".."
    begin
      voidcmd("CDUP")
      return
    rescue FTPPermError => e
      if e.message[0, 3] != "500"
        raise e
      end
    end
  end
  cmd = "CWD #{dirname}"
  voidcmd(cmd)
end