evolve: store obsmarkers to be deleted in evolvestate There are operations which don't create a new node rather they just obsolete one of the unstable node in favour of existing one. In such cases if a further resolution leads to conflicts and user calls `evolve --abort` or `evolve --stop`, we might need to delete that obsmarker. So let's store such obsmarkers in the evolvestate.

evolve: factor out logic to merge branches in separate function This patch moves the logic to merge branches while resolving content-divergence to a separate function. This makes code clear and better to understand.

tests: add more test of --stop while resolving content-divergence This tests the --stop flag when conflicts occur while relocating one of the divergent changesets.