code: func (s *Scanner) readSignature() ([]byte, error) {
  	var sig = make([]byte, 4)
  	if _, err := io.ReadFull(s.r, sig); err != nil {
  		return []byte{}, err
  	}
  
  	return sig, nil
  }
docs: // readSignature reads an returns the signature field in the packfile.