tests/test-dirstate-backup.t

tests: test-parseindex.t works just fine with chg

In 538e850ae737 I annotated tests that failed with chg with "#require no-chg",
but did not investigate why the tests were failing. This test was failing for
unrelated reasons (I forgot to rebuild hg) and so should not be marked as
no-chg.

Test Plan:
  make local
  pushd contrib/chg
  make clean; make
  popd
  cd tests
  for f in '' --with-chg=../contrib/chg/chg; do
    ./run-tests.py --local $f test-parseindex.t
  done

Differential Revision: https://phab.mercurial-scm.org/D3571

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