vim-patch:8.1.1922: in diff mode global operations can be very slow

Problem:    In diff mode global operations can be very slow.
Solution:   Do not call diff_redraw() many times, call it once when redrawing.
            And also don't update folds multiple times.
4f57eefe1e

src/nvim/screen.c

@@ -286,6 +286,11 @@ int update_screen(int type)
     return FAIL;
   }
 
+  // May have postponed updating diffs.
+  if (need_diff_redraw) {
+    diff_redraw(true);
+  }
+
   if (must_redraw) {
     if (type < must_redraw)         /* use maximal type */
       type = must_redraw;