Fix MenuComplete to not use nested edit groups

I didn't feel like properly solving how to handle nested edit groups when
forgetting edits after an undo, but I forget that MenuComplete was
actually using nested edit groups.

MenuComplete doesn't really need nested edit groups, it was just convenient.
I switched to just remembering the edits to remove and saving a single edit
group for the final selection or removing them all if cancelled.

Author: lzybkr

PSReadLine/Completion.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Management.Automation;
@@ -415,7 +417,7 @@ private void PossibleCompletionsImpl(CommandCompletion completions, bool menuSel
 
             if (menuSelect)
             {
-                StartEditGroup();
+                var undoPoint = _edits.Count;
 
                 int selectedItem = 0;
                 bool undo = false;
@@ -502,12 +504,26 @@ private void PossibleCompletionsImpl(CommandCompletion completions, bool menuSel
 
                 WriteBlankLines(displayRows, menuAreaTop);
 
-                EndEditGroup();
+                var lastInsert = ((GroupedEdit)_edits[_edits.Count - 1])._groupedEditItems[1];
+                Debug.Assert(lastInsert is EditItemInsertString, "The only edits possible here are pairs of Delete/Insert");
+                var firstDelete = ((GroupedEdit)_edits[undoPoint])._groupedEditItems[0];
+                Debug.Assert(firstDelete is EditItemDelete, "The only edits possible here are pairs of Delete/Insert");
+
+                var groupEditCount = _edits.Count - undoPoint;
+                _edits.RemoveRange(undoPoint, groupEditCount);
+                _undoEditIndex = undoPoint;
 
                 if (undo)
                 {
                     // Pretend it never happened.
-                    Undo();
+                    lastInsert.Undo();
+                    firstDelete.Undo();
+                    Render();
+                }
+                else
+                {
+                    // Leave one edit instead of possibly many to undo
+                    SaveEditItem(GroupedEdit.Create(new List<EditItem> { firstDelete, lastInsert }));
                 }
             }
             else

PSReadLine/UndoRedo.cs

@@ -175,7 +175,7 @@ public override void Redo()
 
         class GroupedEdit : EditItem
         {
-            private List<EditItem> _groupedEditItems;
+            internal List<EditItem> _groupedEditItems;
 
             public static EditItem Create(List<EditItem> groupedEditItems)
             {