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.

undo-files: also remove the undo.backupfiles The undo.backupfiles is dealt is directly managed by the transaction instead of going through the `localrepo.undofiles`. We start doing minimal management for it before using `cleanup_undo_files` on more situation. Proper handling of it is an intermediate goal of this series.