rhg: fallback when encountering ellipsis revisions

Ellipsis revisions are still experimental and buggy in non-trivial histories. We currently have no plans to improve this situation nor to add support for ellipsis revisions in rhg.

Falling back should be done carefully (since we may have already done some work that is visible to the user), but in this case it's highly unlikely that we're doing anything useful with a repo with ellipsis revisions, so this should be strictly better, also since the error message is more explicit.