evolve: fix the inconsistent behaviour of prune (issue6137)
Let's not update to any revision when working directory parent
is not related to the revision being pruned.
Changes in test file demonstrate the fixed behaviour.
prune: add tests to demonstrate issue6137
Here we can see that prune updates off to the parent revision
even when the pruned revision wasn't related with the working
directory parent.
A follow-up patch will fix this.