vim-patch:8.1.0706: introduce :redrawtabline #10570

Problem:    Tabline is not always redrawn when something that is used in
            'tabline' changes.
Solution:   Add ":redrawtabline" so that a plugin can at least cause the
            redraw when needed.
e12bab3144

src/nvim/screen.c

@@ -6650,7 +6650,7 @@ static void recording_mode(int attr)
 /*
  * Draw the tab pages line at the top of the Vim window.
  */
-static void draw_tabline(void)
+void draw_tabline(void)
 {
   int tabcount = 0;
   int tabwidth = 0;