.arcconfig

changelog: parse copy metadata if available in extras

This lets read back the copy metadata we just started writing. There
are still many places left to teach about getting the copy information
from the changeset, but we have enough ({file_copies}, specifically)
that we can add it now and have some test coverage of it.

Differential Revision: https://phab.mercurial-scm.org/D6186

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    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}