fix(refs): don't assume linux path separator
fix(refs): don't assume linux path separator

Instead, work with os.sep.

Fixes #586

jeblair/GitPython,gitpython-developers/GitPython,jeblair/GitPython,breavo/GitPython,breavo/GitPython,gitpython-developers/gitpython,gitpython-developers/GitPython,gitpython-developers/gitpython