@Override
  public void showDiffForOne(@NotNull AnActionEvent e,
                             @NotNull Project project, @NotNull FilePath filePath,
                             @NotNull VcsFileRevision previousRevision,
                             @NotNull VcsFileRevision revision) {
    GitFileRevision rev = (GitFileRevision)revision;
    Collection<String> parents = rev.getParents();
    if (parents.size() < 2) {
      super.showDiffForOne(e, project, filePath, previousRevision, revision);
    }
    else { // merge
      showDiffForMergeCommit(e, filePath, rev, parents);
    }
  }
@Override
  public void showDiffForOne(@NotNull AnActionEvent e,
                             @NotNull Project project, @NotNull FilePath filePath,
                             @NotNull VcsFileRevision previousRevision,
                             @NotNull VcsFileRevision revision)