It would be more convenient to have a special mode `undo-mode'
that is disabled by default.

To enable undo-mode, it will possible to select from the main menu:

  Edit >
   Enable Undo Mode

Then after enabling it, more menu items will appear on the menu:

  Edit >
   Disable Undo Mode
   Undo
   Redo

Also the current number of undo-limit will be hard-coded
for more convenience:

diff --git a/src/undo.c b/src/undo.c
index 5d705945c4..510d6e8692 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -386,7 +386,7 @@ truncate_undo_list (struct buffer *b)
 	  if (size_so_far > undo_strong_limit)
 	    break;
 	  last_boundary = prev;
-	  if (size_so_far > undo_limit)
+	  if (size_so_far > 160000)
 	    break;
 	}

Does this make sense?

If not, wouldn't be more preferable the following patch
that does the opposite?