.arcconfig

upgrade: use the repository 'ui' as the base for the new repository

The `repo.baseui` contains all the configuration but the one specific to the
repository (so it can be used when dealing with local peer and sub-
repository). However, we need the repository config to be taken into account
when doing the upgrade. Otherwise, the upgrade related config that exists in
the repository config won't be taken into account when performing the update.
A buggy and surprising behavior.

We had to work around protection set around `repo.ui.copy` since we are an
uncommon case.

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