vim-patch:8.1.0064: typing CTRL-W in a prompt buffer shows mode "-- --"

Problem:    Typing CTRL-W in a prompt buffer shows mode "-- --".
Solution:   Set restart_edit to 'A' and check for it.
942b4541a2

Nvim already checked for 'i' in showmode(), so this bug was fixed with <C-W>
(though this patch now changes <C-W> to use 'A').

However, the missing changes I ported for v8.1.0036 use 'A' when a callback
leaves the window in insert mode and edit gets restarted, so this bug was
possible there.

Modify showmode() restart_edit condition to match v8.2.1978:
957cf67d50

src/nvim/window.c

@@ -2240,6 +2240,9 @@ static void leaving_window(win_T *const win)
   // When leaving a prompt window stop Insert mode and perhaps restart
   // it when entering that window again.
   win->w_buffer->b_prompt_insert = restart_edit;
+  if (restart_edit != NUL && mode_displayed) {
+    clear_cmdline = true;  // unshow mode later
+  }
   restart_edit = NUL;
 
   // When leaving the window (or closing the window) was done from a