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.

branchcache: add some blank line in a test This helps each section to stand out.

phases: update the phase set as we go during retract boundary Apparently iterating over the `changed_revs` dictionary is very expensive. On mozilla-try-2019-02-18, a perf::unbundle call with a 10 000 changesets bundle gives give use the following timing. e57d4b868a3e: 4.6 seconds ac1c75188440: 102.5 seconds prev-changeset: 30.0 seconds this-changeset: 4.6 seconds So, the performance regression is gone. Once again: thanks to marvelous Python!

phases: avoid a potentially costly dictionary interation in some case If we retract for the draft phase, there is not non-public item to be retracted and we can skip this part. This part is was apparently super costly thanks to Python. On mozilla-try-2019-02-18, a perf::unbundle call with a 10 000 changesets bundle gives give use the following timing. e57d4b868a3e: 4.6 seconds ac1c75188440: 102.5 seconds this-changeset: 30.0 seconds So we recovered about ⅔ of the regression, the next changeset will give us the rest back.