tests/test-mq-qsave.t

rust-dirstatemap: add `set_cached_mtime` helper method

This will help remove the `get_or_insert` method, which is dangerous because it
does not take the `DirstateMap` counters into account

Differential Revision: https://phab.mercurial-scm.org/D12526

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state