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.