next: properly handle cases when user selects an aspiring child

When there are ambiguous next changesets and only some of them need to be
evolved, and user picks one of them, next used to simply update to it without
evolving. Now we check if it's an aspiring child and evolve it before updating
to it.

In test-prev-next.t, this case was previously handled by a separate hg evolve
--update call. Without this fix, hg next would simply update to the revision 5
("added d", which is still unstable) without evolving it.

Thanks to Sebastian Unger, who spotted this bug on Evolve Testers mailing list.
8 jobs for topic/stable/next-evolve-aspchild in 19 minutes and 11 seconds (queued for 1 second)