Fixes resolving of tag parameter for repo.tag
Fixes resolving of tag parameter for repo.tag

I accessed private variables instead of adding getters,
because other parts of the code do the same and I didn't
know if there was a reason for it.
E.g.: remote.py line 409:
(...) RemoteReference._common_path_default (...)

gitpython-developers/gitpython,gitpython-developers/GitPython,gitpython-developers/GitPython,gitpython-developers/gitpython