undo-files: clean existing files up before writing new one the in the initial design of journal/undo interaction, ages ago, new file always overwrote previous files. This is no longer the case for a long while, so it is time to properly clean things up before writing new ones. Otherwise, inconsistent "undo" state might exist on disk, leading `hg rollback` to misbehave (more that intended).

undo-files: make the undo-prefix configurable in `cleanup_undo_files` The transaction is configuration undo prefix, so we "need" it too.

undo-files: no longer pass the `repo` to `cleanup_undo_files` As foretold in the previous changesets, we no longer need a full repository object here.