next: prompt user to choose child in ambiguity in `hg next --evolve`
This patch makes `hg next --evolve` prompt user to choose a child to evolve and
update when multiple of them exists. This is improvement over previous behavior
where we used to error out saying ambiguous next unstable changeset and told
user to manually use `hg evolve -r rev`. While writing this patch, I am
personally very happy to have this feature as I have encountered this stage many
times and I wanted `next --evolve` to take care of that.
It uses the revselectionprompt thing added in earlier patches.