vim-patch:8.2.4627: flatten() does not use maxdepth correctly

Problem:    flatten() does not use maxdepth correctly.
Solution:   Use a recursive implementation. (closes vim/vim#10020)

acf7d73a7f

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval/funcs.c

@@ -1914,7 +1914,7 @@ static void flatten_common(typval_T *argvars, typval_T *rettv, bool make_copy)
     tv_list_ref(list);
   }
 
-  tv_list_flatten(list, maxdepth);
+  tv_list_flatten(list, NULL, tv_list_len(list), maxdepth);
 }
 
 /// "flatten(list[, {maxdepth}])" function