vfs: extract the audit path logic into a submethod This will make it possible to apply it in more cases.

subrepo-git: use an official origvfs when appropriate The origvfs has the auditor properly set and can move file without issue. The current code is currently working without errors because rename are not audited, yet.

revert: extract origvfs logic in a sub-function The subrepo's "revert" logic could benefit from it.

vfs: treat 'undo.' file the same as 'journal.' file They are the same kind of file, they are protected by the store lock, but directly lives inside the '.hg' directory. No warnings were ever raised about them because `vfs.rename` is not audited. Something we are trying to change.