gdown/gup: add next/previous alias Up and down refer to the customary order in log output rather than concepts; as such, it may be confusing to the user. I for one found them suprising -- they just didn't fit with my mental model. Previous and next, however, imply a clear parent/child relationship. We attroduce the new name as aliases for now.

evolve: allow updating to the successor if the parent is obsolete Although 'hg update' handles this as well, it will also update to tipmost descendent of the successor, if any.