tests/test-dirstate-backup.t

windows: don't consider '$$' to be an escaped '$' when translating to cmd.exe

This functionality was inherited from `os.path.expandvars()`.  But the point of
adding this translating code is to be able to write a portable hook, and bash
wouldn't replace '$$' with '$'.  Escaping with '\' works, and is portable.

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