branchcache: pass the target repository when copying Branchmap are usually copied to be used on a different repoview using a different filter level. Passing the repository around means the repository in `branchcache._repo` will drift from the actual branchmap filter. This is currently "fine" because the repo is only used to retrieve the `nullid` value. However, this is a fairly big trap for any extension or future code using the `_repo` attribute. The replace logic is now using a copy to ensure the right repository view is used to initialized the cached value. We add a couple of assert for make sure this inconsistency does not sneak back.

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