Fix params by appending append_diff_head helper method

Issue link: https://gitlab.com/gitlab-jh/gitlab/-/issues/60

There is a wrong request demo, we don't need diff_head when use diff_id

I created a demo project to show this bug, more detail see: https://gitlab.com/icbd/mr-diff-changes-demo/-/merge_requests/1

And the bug view link: https://gitlab.com/icbd/mr-diff-changes-demo/-/merge_requests/1?diff_id=191040827