code: func (fi *fileIndex) Hashes() []plumbing.Hash {
  	hashes := make([]plumbing.Hash, fi.fanout[0xff])
  	for i := 0; i < int(fi.fanout[0xff]); i++ {
  		offset := fi.oidLookupOffset + int64(i)*20
  		if n, err := fi.reader.ReadAt(hashes[i][:], offset); err != nil || n < 20 {
  			return nil
  		}
  	}
  	return hashes
  }
docs: // Hashes returns all the hashes that are available in the index