fix bookmarks rollback behavior Before this patch undo.bookmarks was created on bookmarks write and not with other transaction-related files. There were two issues: first is that if you have changed bookmarks few times after a transaction happened, rollback will give you a state which can point to non-existing revision. Second is that if you have not changed bookmarks after a transaction, rollback will touch your state anyway. This change also adds `localrepo._writejournal` method, which can be used by other extensions to save their transaction-related backup in right time.