transaction: gather backupjournal logic together in the __init__ The initialisation of file-backup related variable were a bit scattered, we gather them together.

transaction: handle missing file in backupentries (instead of using entries) The case where a backup of a missing file was requested was previously handled by the 'entries' list. As the 'backupentries' is about to gain ability to backup files outside of '.hg/store', we want it to be able to handle the missing file too. Reminder: using 'addbackup' on a missing file means that such file needs to be deleted if we rollback the transaction.