stream-clone-test: simplify the case where server disabled it We have an option to disable it, we don't need to test it with all protocol variants. In addition there is little value in looking at the bytes to bytes details of the reply. Such check is very fragile and consume a lot of time for little value when adjusting formats, caches, and protocol.

rust-matchers: raw regular expression builder Extracting this `re_builder()` from `re_matcher()` makes it reusable in more general cases than matching `HgPath` instances and would help reducing code duplication in RHGitaly.

rust-filepatterns: export glob_to_re function Making this function public should not risk freezing the internal API, and it can be useful for all downstream code that needs to perform glob matching against byte strings, such as RHGitaly where it will be useful to match on branches and tags.

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.