tests/test-dirstate-backup.t

rcutil: don't check if defaultrc/ is a directory -- we know it is

`mercurial/defaultrc/` is a directory both in the Mercurial repo and
once installed on a target platform. The directory was created in
c4ce077588d0 (config: introduce "built-in" default configuration
settings in default.d, 2014-09-04). That commit has some more
information, but it still doesn't seem to say that `defaultrc/` (then
called `default.d/`) could be a file. Perhaps the check was there to
allow you to run the same code on an older install/repo?

Differential Revision: https://phab.mercurial-scm.org/D7624

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