evolve: rename handlenotrouble to _handlenotrouble and add doc As suggested by Pierre-Yves on his comments on f113636997660b807c36cf1d0a06b76d53ada1a4 we make it clear that this function is private and explain what it does.

evolve: improve the help message of --rev User's are not exposed to the notion of revset, this patch makes the help message more intelligible.

evolve: add --rev option to the evolve command This patch is part of a series of patches to refactor the evolve method and make it more readable. We introduce a new --rev flag to specify a revset where to solve the troubles instead of looking from the parent changeset. All add a test checks that --rev works with several commits

evolve: extract cleanup logic in the evolve function We are going to need to reuse the cleanup logic when introducing --rev, so we extract it in a method to avoid code duplication.