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.

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.

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.