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.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.