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.