public @Nullable HgRevisionNumber tip(@NotNull VirtualFile repo) {
    List<HgRevisionNumber> tips = getRevisions(repo, "tip", null, null, true);
    if (tips.size() > 1) {
      throw new IllegalStateException("There cannot be multiple tips");
    }
    if(!tips.isEmpty()) {
      return tips.get(0);
    }
    else return HgRevisionNumber.NULL_REVISION_NUMBER;
  }
public @Nullable HgRevisionNumber tip(@NotNull VirtualFile repo)