vim-patch:8.0.0686: extra redraw when using CTRL-L in second window

Problem:    When typing CTRL-L in a window that's not the first one, another
            redraw will happen later. (Christian Brabandt)
Solution:   Reset must_redraw after calling screenclear().
9f5f7bf4d5

src/nvim/screen.c

@@ -337,6 +337,8 @@ void update_screen(int type)
     screenclear();              // will reset clear_cmdline
     cmdline_screen_cleared();   // clear external cmdline state
     type = NOT_VALID;
+    // must_redraw may be set indirectly, avoid another redraw later
+    must_redraw = 0;
   }
 
   if (clear_cmdline)            /* going to clear cmdline (done below) */