evolve: handle stabilization of merge changeset with both parents obsoleted This patch adds logic to stabilize an orphan merge changeset having both parents obsoleted. The logic tries to first stabilize the merge changeset on successor of second parent and then stabilize the new changeset formed on the successor of first parent. We are stabilizing on second parent first and then to second parent to preserve the first parent's successor as first parent of the merge changeset. Conflicts can occcur and we can loose processing information, therefore we store a variable in statefile `orphanmerge` which represents whether we are processing a merge changeset with both parents obsoleted. Thanks to Pierre-Yves David for suggesting this way and helping to understand the correctness of this. More rigourous test cases for this will be added in next patch.

state: add __setitem__() for cmdstate class Adding __setitem__() will help us in setting the value of variables stored in state files using `state[key] = value` in our code which is very convenient.

evolve: preserve the order of merge parents in `hg evolve --continue` This patch makes sure we preserves the order of merge parents in case of `hg evolve --continue`.

topics: include "operation" metadata in obsmarkers