Multiple points of failures if path to the repository contains unicode

[yarikoptic]

As demonstrated by setting TMPDIR=/tmp/ссылка in #505 . Although some failures might be confounded by having a symlinked portion of the path, but some looked really like failures due to unicode. Here is a direct link to one of the build logs: https://travis-ci.org/gitpython-developers/GitPython/jobs/156924625

[Byron]

A very nice test indeed !
Unfortunately it only seems to affect linux negatively, on OSX I cannot reproduce it with a line like TMPDIR=/tmp/ссылка nosetests git/test/test_commit.py.

I will hold off from that one for a moment though, even though I believe it should definitely be fixed.