code: func (d *DotGit) ObjectPackIdx(hash plumbing.Hash) (billy.File, error) {
  	err := d.hasPack(hash)
  	if err != nil {
  		return nil, err
  	}
  
  	return d.objectPackOpen(hash, `idx`)
  }
docs: // ObjectPackIdx returns a fs.File of the index file for a given packfile