rollback: detect "parentgone" case earlier Detecting this earlier will help us to affect the rollback process sooner, which will help the next changesets. To keep things simple, we degrade the behavior a bit when the `undo.desc` is missing. However since `hg rollback` is not supposed to be used and the `undo.desc` file have been introduced in mercurial 1.6 (2010). I think this is an acceptable evil.

rollback: avoid a `hg commit --addremove` at a critical point The rollback behavior around `hg commit --addremove` has changed slightly. It does not really matters here but keeping that variant out of the way cannot hurt.

rollback: display some graphlog before/after a test piece This make the situation clearer.