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.

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