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.

evolve: strip the relocation commit on `hg evolve --stop` While resolving content-divergence on multiple parents, we relocate one of the commit on the parent of another one and then merge the divergent changesets. Merging can leads to conflicts, and if user does `hg evolve --stop`, we need to strip that relocated changeset too! This patch does that. Test changes demonstrates the fix.