.arcconfig

upgrade: introduce a _copyrevlog method

This function copies a revlog from the old store to the new one, without
re-adding all deltas manually. This will eventually save a lot of time when some
revlog does not needs any conversions.

Code actually using this will be introduced in later changesets.

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    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}