.arcconfig

rebase: use cmdutil.check_at_most_one_arg() for action

Here we also needed to know what the action was (if any), so I've
updated the helper to return any specified option.

Differential Revision: https://phab.mercurial-scm.org/D7640

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