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.

evolve: simplify the evolve function We compute the troubles once and use it in two places instead of recomputing them just to count them.