rollback: remove the dirstateguard usage Thanks to the previous changeset, we no longer needs it. begone !

rollback: explicitly skip dirstate rollback when applicable instead of letting the transaction logic overwrite the dirstate and then overwrite that overwrite with a backup. We simply do not restore the dirstate related file during the _playback call. This open the way to removing the last dirstate guard usage.

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.

rollback: show that the safety works in a associated test That test section is checking that --force is overcoming the safety check, however we did not check that the safety properly detect the situation. We now do for clarity.