transaction: drop per-file extra data support At the moment, transactions support an optional extra data argument for all files to be stored in addition to the original offset. This is used in core only by the revlog inline to external data migration. It is used to memoize the number of revisions before the transaction. That number of can be computed during the walk easily, so drop the requirement. Differential Revision: https://phab.mercurial-scm.org/D9275