code: func OpenFileIndex(reader io.ReaderAt) (Index, error) {
  	fi := &fileIndex{reader: reader}
  
  	if err := fi.verifyFileHeader(); err != nil {
  		return nil, err
  	}
  	if err := fi.readChunkHeaders(); err != nil {
  		return nil, err
  	}
  	if err := fi.readFanout(); err != nil {
  		return nil, err
  	}
  
  	return fi, nil
  }
docs: // OpenFileIndex opens a serialized commit graph file in the format described at
  // https://github.com/git/git/blob/master/Documentation/technical/commit-graph-format.txt