transaction: add a file generation mechanism A new `transaction.addfilegenerator` function is added. It allows external code to register files to be generated. See inline documentation for details. It is important to gather all file creation logic on the transaction as at some point we'll want to mimic the "pre-transaction-commit" logic that we use for revlog. I'm refering to the logic that lets hooks see the result of the transaction before it actually gets committed.

transaction: backup file in a dedicated "namespace" File backup may conflict with other "journal.*" file. We add a fixed part in the backup name file to prevent it.

transaction: use `self.journal` to create backup file Transaction journal name is "journal" in most case, but it can be something else. We use the appropriate attribute to create the file.