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
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