Bug2570 residual: Compact should not change last saved state...

... So that if you save, edit, compact, close without saving, then reopen,
you recover the saved state as expected, not the compacted state.

Implications:

Do not push an undo state for compaction.  Discard redo states instead.

Do not purge undo states after the last saved.  The database still needs them.
So we might reclaim less space than before.

src/UndoManager.cpp

@@ -321,11 +321,7 @@ void UndoManager::PushState(const TrackList * l,
 
    mayConsolidate = true;
 
-   // Abandon redo states
-   if (saved >= current) {
-      saved = -1;
-   }
-   RemoveStates( current + 1, stack.size() );
+   AbandonRedo();
 
    // Assume tags was duplicated before any changes.
    // Just save a NEW shared_ptr to it.
@@ -343,6 +339,14 @@ void UndoManager::PushState(const TrackList * l,
    mProject.QueueEvent( safenew wxCommandEvent{ EVT_UNDO_PUSHED } );
 }
 
+void UndoManager::AbandonRedo()
+{
+   if (saved > current) {
+      saved = -1;
+   }
+   RemoveStates( current + 1, stack.size() );
+}
+
 void UndoManager::SetStateTo(unsigned int n, const Consumer &consumer)
 {
    wxASSERT(n < stack.size());
@@ -438,6 +442,11 @@ void UndoManager::StateSaved()
    saved = current;
 }
 
+int UndoManager::GetSavedState() const
+{
+   return saved;
+}
+
 // currently unused
 //void UndoManager::Debug()
 //{