tests/test-dirstate-backup.t

tests: convert the 'file://\$TESTTMP' rule to an automatic substitution

The rule only triggered on non Windows platforms, even though Windows also
required an adjustment.  Automatic seems better.

The aggressive globbing in test-subrepo-svn.t was found and rewritten by the
substitution.

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