tests/test-dirstate-backup.t

loggingutil: add basic logger backends

These classes will be used in command server. They are similar to
the blackboxlogger, but it can't be factored out since the blackbox is so
tightly coupled with a repo object.

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