Function adopt_legacy_store 

fn adopt_legacy_store(
    app_support: &Path,
    config_dir: &Path,
    path: &Path,
) -> Adoption

Adopt the pre-split machine-wide store into this config dir’s own, once.

Only the default config dir adopts (see is_default_config_dir), and the file is probe-opened first: a store another forge still holds is left where it is rather than renamed behind its back.