1), revlog v2 needs a docket
        file to store that information: since sidedata can be rewritten to the
        end of the data file within a transaction, you can have cases where, for
        example, rev `n` does not have sidedata while rev `n - 1` does, leading
        to `n - 1`'s sidedata being written after `n`'s data.

        TODO cache this in a docket file before getting out of experimental.)