avoid TB in nose tests if there is no tty

GitPython's git.util.get_user_id() calls os.getlogin(), which is
throws an OSError if there is no tty. But GitPython also allows
to avoid the call by setting environment variable
"USER" to anything other than "UNKNOWN".

Therefore we preempt GitPython by calling os.getlogin() ourselves.
If we have to rescue the exception, export the environment var to
avoid traceback:

    Traceback (most recent call last):
      File "/home/sandbox/test/functional/multiproject_tests.py", line 65, in setUp
        TitoGitTestFixture.setUp(self)
      File "/home/sandbox/test/functional/fixture.py", line 129, in setUp
        index.commit('Setting offline.')
      File "/usr/lib/python2.7/site-packages/git/index/base.py", line 887, in commit
        return Commit.create_from_tree(self.repo, tree, message, parent_commits, head)
      File "/usr/lib/python2.7/site-packages/git/objects/commit.py", line 303, in create_from_tree
        committer = Actor.committer(cr)
      File "/usr/lib/python2.7/site-packages/git/util.py", line 354, in committer
        return cls._main_actor(cls.env_committer_name, cls.env_committer_email, config_reader)
      File "/usr/lib/python2.7/site-packages/git/util.py", line 327, in _main_actor
        default_email = get_user_id()
      File "/usr/lib/python2.7/site-packages/git/util.py", line 120, in get_user_id
        username = os.getlogin()
    OSError: [Errno 25] Inappropriate ioctl for device