message: Log when we are doing delta updates
diff: @@ -917,6 +917,7 @@ bool IndexMain_DoCreateIndexUpdate(
  IdMap* previous_id_map = nullptr;
  IndexFile* previous_index = nullptr;
  if (response->previous) {
  + LOG_S(INFO) << "Creating delta update for " << response->previous->file->path;
  previous_id_map = response->previous->ids.get();
  previous_index = response->previous->file.get();
  }
  