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.
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors