repoview: prevent `None` to be passed as the filtername We let such instantiation slip in a previous commit, so we add an explicit check to prevent it to happen in the future.

branchcache: avoid created a `None` filter repoview when writing The repoview class is not intended to be used for unfiltered repository.

rust-index: don't use mutable borrow to computed filtered heads This does not need to mutate the index. This is the prime suspect for some RuntimeError raised during some pushes.