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.