evolve: store the skippedrevs in evolvestate
While doing `hg evolve`, there are some revs which are skipped as they can't be
evolved. Before this patch, we did not store this information in evolvestate and
neither they were evolved, so `hg evolve --continue` will again try to stabilize
those revisions. As much time as we run `hg evolve --continue`, this will
happen which is not a good behavior.
This patch adds a skippedrevs list to evolvestate where we store such revisions
so that we don't try to stabilize them again.