.arcconfig

rust: Update the memmap2 crate to version 0.4.0

This version:

* Optionally implements the `StableDeref` trait, removing the need
  for a wrapper type doing that
* Adds support for mapping empty files, which can routinely happen
  for dirstate-v2 data files. This was a cause of some failing tests
  when running with `run-tests.py --rhg --extra-config-opt rhg.status=1`

Differential Revision: https://phab.mercurial-scm.org/D11446

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