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 ()
 {

src/TDB2.h

@@ -136,6 +136,7 @@ public:
   bool read_only ();
 
   int num_local_changes ();
+  int num_reverts_possible ();
   size_t data_size ();
 
   void clear ();
@@ -154,11 +155,12 @@ private:
 public:
   TF2 pending;
   TF2 completed;
-  TF2 undo;
 
 protected:
   friend class CmdSync; // CmdSync accesses the backlog directly
   TF2 backlog;
+  friend class CmdInfo; // CmdInfo uses undo data to give history
+  TF2 undo;
 
 private:
   std::string        _location;

src/commands/CmdStats.cpp

@@ -65,9 +65,8 @@ int CmdStats::execute (std::string& output)
   // Go get the file sizes.
   size_t dataSize = Context::getContext ().tdb2.data_size ();
 
-  // Count the undo transactions.
-  std::vector <std::string> undoTxns = Context::getContext ().tdb2.undo.get_lines ();
-  int undoCount = std::count(undoTxns.begin(), undoTxns.end(), "---");
+  // Count the possible reverts.
+  int undoCount = Context::getContext ().tdb2.num_reverts_possible ();
 
   // Count the backlog transactions.
   int numLocalChanges = Context::getContext ().tdb2.num_local_changes ();