relnotes: add 6.4.4

revlog: avoid possible collision between directory and temporary index Since 6.4, we create a temporary index file to write the split data without overwriting the inline version too early. However, the store encoding does not prevent these new `.i.s` file to collide with a directory with the same name. While the odds for such a collision to happens are fairly low, the collision would prevent Mercurial from working. The store encoding have a mitigation solution in place to prevent such collisions from happening for `.i` and `.d` files, but not for other extensions. We cannot update this encoding scheme to solve the issue since it would diverge from older version of Mercurial. Instead, we create an alternative directory tree dedicated to such files. The use of the `.i` extension combined with store encoding will prevent collisions there.

revlog: move the computation of the split_index path in a property This is about to become more complex, so we gather the logic in a single place.

rust-dirstate: fall back to v1 if reading v2 failed This will help us not fail when a v1 dirstate is present on disk while a v2 was expected (which could happen with a racy/interrupted upgrade).

dirstate: add test showing dirstate version mismatch causes an error We should fall back to trying dirstate v1 when v2 fails to read.

rust-dirstate: rename `has_dirstate_v2` to `use_dirstate_v2` It is closer to the right semantics. I added a docstring to better explain the reasonning. In the next patch(es), I will address the underlying issue of finding the "wrong" version of the dirstate on disk.

delta-find: fix pulled-delta-reuse-policy=forced behavior The code that select delta still has too many oportunity to discard the delta is has been forcibly asked to reuse. However is is fairly easy to use a dedicated fastpath for this case. So we do so. Cleaning other code that tries to enforce that policy will be done on default.

delta-find: display more information about the search in some case This will be useful to access the effect of the delta reuse policy.

deltafind: issue debug information when we fast-path rivial case too More debug options never hurts.

delta-find: gather the condition to blindly use a full snapshot together We are about to make the `if` body bigger, so having only one of them is simpler/