branching: merge stable into default This also added the small fix need in Rust tests for the new `DirstateMap::pack_v2` API change in stable.

rust-dirstate-v2: clean up previous data file after the docket is written This was overlooked before and caused many data files to keep living forever. We could potentially consider adding a random cleanup of them in case they slipped through one day. Differential Revision: https://phab.mercurial-scm.org/D12581