revlog: make a predicate clearer with parens

transaction: extract backupentry registration in a dedicated function We are about to use the 'backupentry' mechanism to allow cleaning up transaction-related temporary files (such as 'changelog.i.a'). We start by extracting the entry registration into its own method for easy reuse. At that point, I would like to rename the backup-file related variable to something generic but I'm a bit short of ideas.

transaction: pass the transaction to 'postclose' callback This mirrors the API for 'pending' and 'finalize' callbacks. I do not have immediate usage planned for it, but I'm sure some callback will be happy to access transaction related data.

transaction: pass the transaction to 'finalize' callback The callback will likely need to perform some operation related to the transaction (eg: registering file update). So we better pass the current transaction as the callback argument. Otherwise callback that needs it has to rely on horrible weak reference trick. This allow already allow us to slay a wild weak reference usage.

transaction: pass the transaction to 'pending' callback The callback will likely need to perform some operation related to the transaction (eg: backing files up). So we better pass the current transaction as the callback argument. Otherwise callback that needs it has to rely on horrible weak reference trick. The first foreseen user of this is changelog._writepending. We would like it to register the temporary file it create for cleanup purpose.

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.