code: func (m *indexFileMap) Add(sha string, index *indexFile) {
  	m.mutex.Lock()
  	defer m.mutex.Unlock()
  
  	pairKey := strings.Join([]string{sha, index.Name}, ":")
  	if m.nameShaPairs[pairKey] {
  		return
  	}
  
  	m.nameMap[sha] = append(m.nameMap[sha], index)
  	m.nameShaPairs[pairKey] = true
  }
docs: // Add appends unique index files to the given SHA, "sha". A file is considered
  // unique if its combination of SHA and current filename have not yet been seen
  // by this instance "m" of *indexFileMap.