transaction: pass the name of the "undo" journal to the transaction It is time for the transaction to be responsible for setting up the undo data. It is necessary to move this logic into the transaction because many more files are handled now, and the transaction is the object tracking them all. The value can be set to None if no undo should be set.

rollback: have an empty entry for the vfsmap in rollback This empty string key is used for the store. This will be needed to properly rollback backup in a future changesets.