.arcconfig

upgraderepo: allow extension to register preserved requirements

Some requirement does not directly result from config and needs more advanced
logic to be preserved. The current example is 'largefiles'. We add a hook
point in the upgrade code so that extensions can handle these cases.

The 'largefiles' extension will use it in the next changeset.

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