@NotNull
  private DiffFromHistoryHandler getDiffHandler() {
    VcsHistoryProvider historyProvider = myActiveVcs.getVcsHistoryProvider();
    DiffFromHistoryHandler handler = historyProvider != null ? historyProvider.getHistoryDiffHandler() : null;
    return handler != null ? handler : new StandardDiffFromHistoryHandler();
  }
@NotNull
  private DiffFromHistoryHandler getDiffHandler()