[textinput] Undo buffer misses one entry in the actual sequence

[ferdymercury]

Describe the bug

The undo shortcut in the prompt editor misses one entry in the sequence (I guess it is not added to the buffer)

Expected behavior

Entries where ENTER are pressed are also added to the undo buffer.

To Reproduce

root -l
root [0] int x
(int) 0
root [1] float y
(float) 0.00000f
// Start then pressing repeatedly CTRL+_

--> Neither x nor y are shown when undoing.

Setup

   ------------------------------------------------------------------
  | Welcome to ROOT 6.27/01                        https://root.cern |
  | (c) 1995-2021, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Jan 12 2022, 10:17:19                 |
  | From heads/master@v6-25-01-2870-gdac9b6398d                      |
  | With c++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0                     |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

Additional context

#10121

[jalopezg-git]

Actually, I think that the correct behavior should be to clear the undo history after accepting a new input line (which also follows what GNU-readline does).