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.

rust-index: don't use mutable borrow for head-diff computation It does not needs to mutate the index. This is one of the two suspects of RuntimeError being thrown during push.

branchcache: move head writing in a `_write_headers` method Same rational: this will help having format variants.

branchcache: move head writing in a `_write_heads` method Same rational: this will help having format variants.

branchcache: move the header loading in a `_load_header` class method This will help changing header parsing in format variants.

branchcache: simplify a long line Gratuitous change to help code readability.

branchcache: rename `load` to `_load_heads` We are about to have more similar function, we rename the existing one to a more meaningful name and mark it private in the process.

branchcache: move the filename to a class attribute This prepare the introduction of more variant of cache.