code: func (r *Rewriter) cacheCommit(from, to []byte) {
  	r.mu.Lock()
  	defer r.mu.Unlock()
  
  	r.commits[hex.EncodeToString(from)] = to
  }
docs: // cacheEntry caches then given "from" commit so that it is always rewritten as
  // a *git/gitobj.Commit equivalent to "to".