vim-patch:8.2.0935: flattening a list with existing code is slow

Problem:    Flattening a list with existing code is slow.
Solution:   Add flatten(). (Mopp, closes vim/vim#3676)
077a1e670a

src/nvim/eval/typval.c

@@ -640,6 +640,53 @@ tv_list_copy_error:
   return NULL;
 }
 
+/// Flatten "list" in place to depth "maxdepth".
+/// Does nothing if "maxdepth" is 0.
+///
+/// @param[in] list       List to flatten
+/// @param[in] maxdepth   Maximum depth that will be flattened
+///
+/// @return OK or FAIL
+int tv_list_flatten(list_T *list, long maxdepth)
+  FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  listitem_T *item;
+  int n;
+  if (maxdepth == 0) {
+    return OK;
+  }
+
+  n = 0;
+  item = list->lv_first;
+  while (item != NULL) {
+    fast_breakcheck();
+    if (got_int) {
+      return FAIL;
+    }
+    if (item->li_tv.v_type == VAR_LIST) {
+      listitem_T *next = item->li_next;
+
+      tv_list_drop_items(list, item, item);
+      tv_list_extend(list, item->li_tv.vval.v_list, next);
+
+      if (item->li_prev == NULL) {
+        item = list->lv_first;
+      } else {
+        item = item->li_prev->li_next;
+      }
+
+      if (++n >= maxdepth) {
+        n = 0;
+        item = next;
+      }
+    } else {
+      n = 0;
+      item = item->li_next;
+    }
+  }
+  return OK;
+}
+
 /// Extend first list with the second
 ///
 /// @param[out]  l1  List to extend.