Make TDB2.backlog non-public

The stats command gets this information from an API that will also work
for TaskChampion.  The sync command still accesses the field directly,
as the command must be completely rewritten for TaskChampion.

src/TDB2.cpp

@@ -1259,6 +1259,19 @@ bool TDB2::read_only ()
          ;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+int TDB2::num_local_changes ()
+{
+  std::vector <std::string> lines = backlog.get_lines ();
+  return std::count_if(lines.begin(), lines.end(), [](const auto& line){ return line.front() == '{'; });
+}
+
+////////////////////////////////////////////////////////////////////////////////
+size_t TDB2::data_size ()
+{
+  return pending._file.size () + completed._file.size () + undo._file.size () + backlog._file.size ();
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 void TDB2::clear ()
 {