undo-files: drop the old undo rename logic It is no longer necessary I am not changing the transaction.__init__ signature since we are on stable right now.

undo-files: have the transaction directly tracks and manages journal rename This is much simpler this way.

undo-files: add a undoname closure to the _write_undo method We will also needs it when the transaction will take care of the other journal files, which is soon™.

undo-files: cleanup backup when cleaning undos Previously, the backups were left behind, by operation cleaning the undo's like strip, narrow and stream clone. The remaining elevant in the room is the transaction itself, who does not properly cleanup undo backup before copying the new ones.

undo-files: factor the vfs map in a repository property We define it in multiple locations and inconsistencies are appearing. So we now have a single definition point.

undo-files: add a utility function to read the backup-files definition We will need it in multiple places. so lets factor the logic around.

undo-files: use the cleanup function in streamclone Lets use the same code, so that we can fix things only once.