feat: cmdheight=0 #16251

Fix https://github.com/neovim/neovim/issues/1004

Limitation: All outputs need hit-enter prompt.

Related:
https://github.com/neovim/neovim/pull/6732
https://github.com/neovim/neovim/pull/4382

src/nvim/normal.c

@@ -2747,6 +2747,10 @@ void pop_showcmd(void)
 
 static void display_showcmd(void)
 {
+  if (p_ch < 1 && !ui_has(kUIMessages)) {
+    return;
+  }
+
   int len;
   len = (int)STRLEN(showcmd_buf);
   showcmd_is_clear = (len == 0);