tests/test-dirstate-backup.t

config: rename allow_archive to allow-archive

As part of ConfigConsolidationPlan [1], rename the option according to
the new UI guidelines [2] and add an alias for backward compatibility.

[1]: https://www.mercurial-scm.org/wiki/ConfigConsolidationPlan
[2]: https://www.mercurial-scm.org/wiki/UIGuideline#adding_new_options

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
  [255]

No dirstate backups are left behind

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