Fix treesit-induce-sparse-tree

Fix treesit-induce-sparse-tree

Because not-at-all-werid way we implemented ts_build_sparse_tree, it’s
return value needs a bit post-processing (i.e., reverse its cdr).

* src/treesit.c (Ftreesit_induce_sparse_tree): Reverse the top-level
children list.
(ts_build_sparse_tree): Add comment.