Resolve #6804: v2-update reports new index-state

For example:

    Downloading the latest package list from hackage.haskell.org
    Updated package list of hackage.haskell.org to index-state 2020-05-15T09:13:06Z
    To revert to previous state run:
        cabal v2-update 'hackage.haskell.org,2020-05-10T18:58:02Z'

Or

    Package list of hackage.haskell.org is up to date at index-state 2020-05-15T09:13:06Z