fix(redraw): make sure :redraw! redraws command line

In Vim :redraw! uses CLEAR, which calls screenclear(), which sets
redraw_cmdline. In Nvim :redraw! uses NOT_VALID, which does not set
redraw_cmdline automatically, so set it explicitly.

src/nvim/ex_docmd.c

@@ -6994,6 +6994,7 @@ static void ex_redraw(exarg_T *eap)
   update_topline(curwin);
   if (eap->forceit) {
     redraw_all_later(NOT_VALID);
+    redraw_cmdline = true;
   }
   update_screen(eap->forceit ? NOT_VALID
                 : VIsual_active ? INVERTED : 0);