rebase: properly calculate descendant set when aborting (issue3332) Checking for descendants of target being public was also wrong.

merge with stable

strip: ignore -n (issue3326) (BC) -n could be confused for --dry-run by foolhardy users, resulting in permanent data loss. As leaving a backup when none was requested is significantly less disastrous, the short option is silently ignored. Old scripts continue to work, users only get lightly burned.

aliases: use empty string for missing position parameters (issue3331)