make TDB2.undo non-public

The stats command calls an API to provide this information in a way that
will still be relevant for TaskChampion, while CmdInfo's access to the
data remains.  The TaskChampion interface for per-task hitsory is still
not ready.

src/TDB2.cpp

@@ -1266,6 +1266,13 @@ int TDB2::num_local_changes ()
   return std::count_if(lines.begin(), lines.end(), [](const auto& line){ return line.front() == '{'; });
 }
 
+////////////////////////////////////////////////////////////////////////////////
+int TDB2::num_reverts_possible ()
+{
+  std::vector <std::string> lines = undo.get_lines ();
+  return std::count(lines.begin(), lines.end(), "---");
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 size_t TDB2::data_size ()
 {