.arcconfig

import: use context manager for lock, dirstateguard, transaction

A tiny side-effect is that the transaction is now closed after saving
the commit message.

Differential Revision: https://phab.mercurial-scm.org/D3748

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