branchcache: add a "pure topological head" fast path In a narrow but actually quick common case, all topological heads are all on the same branch and all open. In this case, computing the branch map is very simple. We can quickly detect situation where this situation will not change. So we update the V3 format to be able to express this situation and upgrade the update code to detect we remains in that mode. The branch cache is populated with the actual value when the branch map is accessed, but the update_disk method can do the update without needing to populate it.

branchcache: move the processing of the new data in a dedicated method In a future changeset, this will allow the V3 of the branch cache to use a fast path when possible.

branchcache: gather newly closed head in a dedicated set This is part of a series to more clearly split the update in two step. This will allow us to introduce a fast path during update in a future changeset.

branchcache: gather new obsolete revision in a set This is part of a series to more clearly split the update in two step. This will allow us to introduce a fast path during update in a future changeset.

branchcache: filter obsolete revisions sooner Since we won't do anything with the obsolete revisions, we can just ignore them sooner.

branchcache: skip entries that are topological heads in the on disk file In the majority of cases, topological heads are also branch heads. We have efficient way to get the topological heads and efficient way to retrieve their branch information. So there is little value in putting them in the branch cache file explicitly. On the contrary, writing them explicitly tend to create very large cache file that are inefficient to read and update. So the branch cache v3 format is no longer including them. This changeset focus on the format aspect and have no focus on the performance aspect. We will cover that later.

branchcache: simplify the branch rev cache test We don't need that many content dump and this gets in the way in change in access pattern (e.g. accessing revision in a different order change the order of branches in the "names" file). So we simplify this test in advance.

branchcache: store filtered hash and obsolete hash independently for V3 This will avoid the bug covered in tests/test-branches-obsolete.t when we stop storing all heads explicitly in V3.

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.