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.

localrepo.commit: abort if merge driver's conclude step hasn't been run yet This case also requires 'hg resolve --all' to be run before continuing.

merge.mergestate: add a way to get the merge driver state This will be useful to check what the status of the merge driver is.

merge.mergestate: add a way to get the other side of the merge It's surprising there was no API at all for this until now. In any case this will be needed from custom merge drivers.

commands.resolve: support printing out driver-resolved files There's no user-visible way to mark files as driver-resolved, so this status won't be visible.