merge: simplify mergestate iter

resolve: simplify "finished" message The recently introduced message was: no unresolved files; you may continue your unfinished operation This had three problems: - looks a bit like an error message because it's not saying "we've just resolved the last file" - refers to "unfinished operation", which won't be the case with "update" or "merge" - introduces semicolons to error messages, which is stylistically questionable I've simplified this to: no more unresolved files In the future, if we want to prompt someone to continue a particular operation, we should use a hint style: no more unresolved files (use 'hg graft --continue' to finish grafting)