tests/test-dirstate-backup.t

py3: decode bytes before logging in run-tests.py

Avoids messages like "Found prerequisite b'diff' at b'/usr/bin/diff'"
under Python 3.

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