Fix undo bug

There was a bug in undo code where if you undid some stuff, made an edit,
then started a grouped edit, the undo state was incorrect.  A simple
repro was something like (in Windows mode)

    a b c d Ctrl+z Ctrl+z Ctrl+z Shift+LeftArrow Ctrl+x

PSReadLine/ReadLine.cs

@@ -32,7 +32,7 @@ namespace PSConsoleUtilities
         private readonly StringBuilder _statusBuffer;
         private string _statusLinePrompt;
         private List<EditItem> _edits;
-        private readonly Stack<int> _pushedEditGroupCount;
+        private int _editGroupStart;
         private int _undoEditIndex;
         private int _mark;
         private bool _inputAccepted;
@@ -359,8 +359,6 @@ namespace PSConsoleUtilities
             _savedCurrentLine = new HistoryItem();
             _queuedKeys = new Queue<ConsoleKeyInfo>();
 
-            _pushedEditGroupCount = new Stack<int>();
-
             string hostName = null;
             try
             {
@@ -394,7 +392,7 @@ namespace PSConsoleUtilities
             _buffer.Clear();
             _edits = new List<EditItem>();
             _undoEditIndex = 0;
-            _pushedEditGroupCount.Clear();
+            _editGroupStart = -1;
             _current = 0;
             _mark = 0;
             _emphasisStart = -1;

PSReadLine/UndoRedo.cs

@@ -14,6 +14,11 @@ namespace PSConsoleUtilities
             if (removeCount > 0)
             {
                 _edits.RemoveRange(_undoEditIndex, removeCount);
+                if (_editGroupStart >= 0)
+                {
+                    // Adjust the edit group start if we are started a group.
+                    _editGroupStart -= removeCount;
+                }
             }
             _edits.Add(editItem);
             _undoEditIndex = _edits.Count;
@@ -21,16 +26,21 @@ namespace PSConsoleUtilities
 
         private void StartEditGroup()
         {
-            _pushedEditGroupCount.Push(_edits.Count);
+            if (_editGroupStart != -1)
+            {
+                // Nesting not supported.
+                throw new InvalidOperationException();
+            }
+            _editGroupStart = _edits.Count;
         }
 
         private void EndEditGroup()
         {
-            var groupEditStart = _pushedEditGroupCount.Pop();
-            var groupEditCount = _edits.Count - groupEditStart;
-            var groupedEditItems = _edits.GetRange(groupEditStart, groupEditCount);
-            _edits.RemoveRange(groupEditStart, groupEditCount);
+            var groupEditCount = _edits.Count - _editGroupStart;
+            var groupedEditItems = _edits.GetRange(_editGroupStart, groupEditCount);
+            _edits.RemoveRange(_editGroupStart, groupEditCount);
             SaveEditItem(GroupedEdit.Create(groupedEditItems));
+            _editGroupStart = -1;
         }
 
         /// <summary>