evolve: write our own custom evolvestate file Since for ever, we were using 'graftstate' to record the node currently being evolve and allow 'hg evolve --continue' we now move to our on 'evolvestate' file. This remove and issue with 'hg summary' listing interrupted evolve as graft. This also open the way for storing more data into that file and allow proper --abort and --continue of the whole evolve operation (and not just the last one). The whole thing is very hacky but at least there is some progress. Thanks goes to Shusen Liu for initiating this work.

readme: update readme for issue 4966