Support quarantining a localrepo.Repo

Currently there's no way to quarantine a localrepo instance one already has available. Applying a quarantine to an existing Repo instance is preferable to building a new one with a quarantined GitRepo as the latter would require the call site to know how to build a localrepo instance. This commit implements Quarantine method on the localrepo to apply a quarantine directory to it. It returns a new localrepo instance which refers to the same repository although with the quarantine directory applied.

Part of #4790 (closed)