FIX #535: expand also GIT_DIR var on Repo-construct
FIX #535: expand also GIT_DIR var on Repo-construct

+ Ignore "empty" GIT_DIR vars.
+ Improve documentation on the constructor `path` parameter.
gitpython-developers/gitpython,gitpython-developers/GitPython,breavo/GitPython,breavo/GitPython,nvie/GitPython,nvie/GitPython,jeblair/GitPython,jeblair/GitPython,gitpython-developers/GitPython,gitpython-developers/gitpython