branchcache: show the cache file content in test-branches-obsoletes.t This help to track the changes in format between v2 and v3.

branchcache: rework the `filteredhash` logic to be more generic We now have a more flexible `key_hashes` tuple. We duplicated various logic in the V2 and V3 version of the cache as the goal is to start changing the logic for V3 in the next few changesets.

filteredhash: rename the filteredhash function The new name is less ambiguous, as we are about to introduce an alternative function it seems like a good idea to have clearer name to distinct the two.

filteredhash: split the computation of revision sets The branch2's filteredhash combines the filtered revisions and the obsolete ones, this will creates issue for implicit reference to heads we want to introduce for the v3 of the branch cache format. So we isolate this logic for alternative use.

filteredhash: move the hashing in its own function This will help us to reuse this logic in variants of the hashes used for branch cache validation.

branchcache: cleanup the final key generation after update A lot of duplicated work seemed to be done, as we already update the tiprev and tipnode when needed right before. So we simplify that part to focus on the filtered hash. See inline comment for details.

branchcache: add more test for the logic around obsolescence and branch heads While working on branch-cache-v3, we noticed some ambiguity in the filtered+obsolete hash. However this was only caught by a rebase test by chance. It seems important to explicitly tests these cases.

branchcache-v3: use more explicit header line The key-value approach is clearer and gives more rooms to have the format evolve in a clear way. It also provides extension (like topic) simpler way to extend the validation scheme. This is just a small evolution, the V3 format is still a work in progress.

branchcache-v3: introduce a v3 format For now the format is the very same, however we will start changing it in future changesets.

branchcache: use an explicit class for the v2 version This prepare the introduction of an experimental v3 format version. In the process, we move the description of the format in that new class.