fix(cmd): safely read from stderr
fix(cmd): safely read from stderr

Fixes #383

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