Function open_repo_at 

pub(crate) fn open_repo_at<ObjectID>(
    path: &Path,
    insecure: bool,
    require_verity: bool,
    no_upgrade: bool,
) -> Result<Repository<ObjectID>>
where
    ObjectID: FsVerityHashValue,

Open a repo at an explicit path, auto-upgrading old-format repos unless no_upgrade is set.

This is the parameterized core shared by open_repo (which derives the path and flags from App) and the varlink service (which holds these values directly).