commands.resolve: print out warning when only driver-resolved files remain 'hg resolve --all' will be the canonical way to run the 'conclude' step of the merge driver.

merge.mergestate: set merge driver state to 's' if there's none present This allows mergestate.mdstate() to return 's' if there's nothing to be done on the merge driver end.

merge.mergestate: only check for merge driver when property is accessed Otherwise 'hg update --clean', 'hg rebase --abort' etc wouldn't work.