tests/test-dirstate-backup.t

tests: handle string escaping/encoding on Python 3

This code was failing on Python 3 for a few reasons:

1) sys.argv is str and str doesn't have a .decode()
2) the "string_escape" encoding was renamed to "unicode_escape"

It is wonky casting to bytes to str to bytes. But this is test
code, so meh. I don't believe we exercise any code paths in these
tests where the arguments aren't ascii.

Differential Revision: https://phab.mercurial-scm.org/D5667

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