next: factor out the logic to update to changeset in another function
This patch moves the logic to update to children changeset during `hg next` in a
separate function because we want to re-use the logic when we will prompt user
to choose a changeset in case of multiple childrens.
utility: add a function to prompt user to choose a revision
This patch adds a utility function to interactively prompt user to choose a
revision to proceed in cases when there are multiple revisions which are
candidates.
The function will be used in `hg evolve`, `hg next`, `hg prev` etc. If user
feedback will be good, I plan to move it to core and use it at more possible
places.