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/fold.c

@@ -771,6 +771,11 @@ void foldUpdate(win_T *wp, linenr_T top, linenr_T bot)
     return;
   }
 
+  if (need_diff_redraw) {
+    // will update later
+    return;
+  }
+
   // Mark all folds from top to bot as maybe-small.
   fold_T *fp;
   (void)foldFind(&wp->w_folds, top, &fp);