fix(treesitter): don't update fold if tree is unchanged

Problem:
Folds are opened when the visible range changes even if there are no
modifications to the buffer, e.g, when using zM for the first time. If
the parsed tree was invalid, on_win re-parses and gets empty tree
changes, which triggers fold updates.

Solution:
Don't update folds in on_changedtree if there are no changes.

runtime/lua/vim/treesitter/_fold.lua

@@ -299,7 +299,9 @@ local function on_changedtree(bufnr, foldinfo, tree_changes)
       local srow, _, erow = Range.unpack4(change)
       get_folds_levels(bufnr, foldinfo, srow, erow)
     end
-    foldupdate(bufnr)
+    if #tree_changes > 0 then
+      foldupdate(bufnr)
+    end
   end)
 end