next: prompt user to choose a children in case of ambiguity
This patch adds functionality to prompt user and ask the children to which next
should move to incase of multiple childrens. This is a very nice UI because
otherwise user has to lookup changeset hash, and then manually do `hg update` to
that. In this case we show hash and description so user can choose which one to
update easily.
It is using the interactive revision choosing functionality added in a previous
patch.