prev: prompt user to choose parent in case of multiple parents This patch adds functionality to `hg prev` to prompt user to choose one parent to update when multiple parents exist. Surprisingly there were no tests for this case before this patch, I added one with this patch.

next: cleanup logic around return by returning early In the cmdnext function, there are some conditionals which returns early and some which sets a result value to be returned later. Let's return from all the conditionals early. There are if, elif and else involved, so it's sure we will end up in one of the conditional, each conditional will return, so it's guranteed that we are returning the correct value and returning everytime.

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.

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.

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.

tests: add a test about evolve stabilizing orphan caused by splits There are some similar tests lying around in the test suite but they are not well arranged. I wanted the test suite to be a goto reference for all the behaviors till the time we get all of them documented.