evolve: add an help topic about evolution The text is probably bad, but this have been laying around for quite sometime (with Mercurial core in mind) lets move forward and add this to evolve.

evolve: remove _picknexttroubled It was not used anymore

evolve: warn about every skipped evolution When we could not evolve a revision, we used to silently ignore it. We now inform the user that we ignored something and tell him why (with more or less informative message).