fix(git-test): assure test does works on linux

It shows that the previous implementation was never really working on
linux, and thus failed on travis as well for good reason.

Closes gitpython-developers#303

Author: Byron

git/test/test_git.py

@@ -199,7 +199,8 @@ def test_environment(self, rw_dir):
                 try:
                     remote.fetch()
                 except GitCommandError as err:
-                    assert 'FOO' in str(err)
+                    assert 'ssh-origin' in str(err)
+                    assert err.status == 128
                 # end
             # end
         # end if select.poll exists