in 0.3.2 executing git commands using environment variables breaks

[electrofelix]

Recent release 0.3.2 contains a change to set the LC_MESSAGES=C for all git commands to ensure the error messages are consistent across systems. this breaks commands that use the environment to allow users to control behaviour.

To reproduce, clone the GitPython repo, checkout the current 0.3 branch and within execute the following command

PYTHONPATH=. python -c 'import os
import git
test = git.Git(".")
os.environ["GIT_EDITOR"] = "cat"
test.var("GIT_EDITOR")'

Output will be:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "git/cmd.py", line 240, in <lambda>
    return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
  File "git/cmd.py", line 536, in _call_process
    return self.execute(make_call(), **_kwargs)
  File "git/cmd.py", line 399, in execute
    raise GitCommandError(command, status, stderr_value)
git.exc.GitCommandError: 'git var GIT_EDITOR' returned exit status 128: fatal: Terminal is dumb, but EDITOR unset

This breaks https://github.com/stackforge/git-upstream usage of GitPython.