transaction: pass the transaction to 'pending' callback...
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.
