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.
