evolve: store the obsmarkers to be deleted in evolvestate Sometimes, we don't create a new node and obsolete one of the old node in favour of an existing node. When user calls, `hg evolve --abort`, we need to strip that obsmarker to go back in the same old state. The obsmarker created for new node are stripped when the new nodes are stripped. This patch start storing such obsmarkers in the evolvestate so that we can delete them later if required. Right now we just store successsor and predecessor information, we might need a better obsmarker serialization techinque in future to make it more robust.