dirstate: remove _actual_file_name methode This is dead-code

undo-files: cleanup legacy files when applicable The "journal" code is much more compact in 6.4, and so is the "undo" files as a result. However the previous version were much noisier, so let us cleanup undo files from older version too.

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.

undo-files: relies on a explicit list of possible undo files Instead of infering the list of undo files from the `_journalfiles` method on `localrepository`, we explicitly have the list of file in a constant next to the cleanup code. In practice this does not change much as `_journalfiles` is already returning the same "static" list and no internal or extensions extensions seems to actually wrap that. In addition, that list is not "too short" for cleanup, in case we need to cleanup undo files from older version of Mercurial that used to use more of them. this will be dealt with in a later changesets. This change is a step toward our goal to use the `cleanup_undo_files` within the transaction. The transaction has no reference to the `repo` object, so we need to move toward `cleanup_undo_files` not having one either.

undo-files: move the undo cleanup code in the transaction module Now that undo creation is gathered in the transaction module, let us move the code cleaning them up there too. This will be useful to better clean previous undo files up before creating new ones.

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™.