tests/test-dirstate-backup.t

dirstatemap: rename `_rustmap` to `_map`

This match the name of the `map` for the other implementation and will make it
simpler to share code between the two.

Differential Revision: https://phab.mercurial-scm.org/D11566

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [10]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate