next: move logic to evolve the children to it's own function This patch moves the logic which evolves the children and updates to it in `hg next --evolve` to it's own function as we want to use that again.

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.

next: show changeset hash in `--dry-run` instead of rev numbers Rev numbers can change, hence we should show rev hash instead of rev numbers in dry-run.

evolve: make sure we consider all cases in if-else The part of code which is touched decides on evolution of merge changeset. We need proper handling of each case there, so before adding logic, let's add conditional for each case so that upcoming patches are easy to understand.

tests: add more cases about stabilization of an orphaned merge commit This patch adds three more cases related to stabilization of orphaned merge commits.

evolve: fix missing negation Spotted by Pulkit Goyal.

evolve: clarify message for ambiguous destination from split Message is a bit long, but should be less scary.

evolve: prompt user to select destination for stabilization in case of split This patch adds functionality to prompt user to select a revision when there exists multiple stabilization destination because of split on multiple branches. Test are also added for the feature. I am not sure whether this is the best message which we can show, if people have suggestions I will like to improve the messages shown in prompt.

tests: remove some tests which are moved to dedicated files The tests in test-unstable.t files are now moved to their dedicated file with all the cases to that in that file. Let's delete them from here.

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.