Determine Git directory correctly

`git rev-parse --git-dir` returns the *absolute* path (unless `$GIT_DIR`
was set to something like `.` ).

    Installing geocommit hook in
    /home/grawity/src/geocommit//home/grawity/src/geocommit/.git/hooks/post-rewrite
    Traceback (most recent call last):
      File "/home/grawity/.local/bin/git-geo", line 9, in <module>
        load_entry_point('geocommit==0.9.3beta1', 'console_scripts', 'git-geo')()
      File "/home/grawity/.local/lib/python2.7/site-packages/geocommit/__init__.py", line 300, in git_geo
        f(sys.argv[2:])
      File "/home/grawity/.local/lib/python2.7/site-packages/geocommit/__init__.py", line 165, in cmd_setup
        self.install_hooks(".")
      File "/home/grawity/.local/lib/python2.7/site-packages/geocommit/__init__.py", line 69, in install_hooks
        self.install_hook(git_dir, hook, code)
      File "/home/grawity/.local/lib/python2.7/site-packages/geocommit/__init__.py", line 90, in install_hook
        f = open(hook, "w")
    IOError: [Errno 2] No such file or directory: '/home/grawity/src/geocommit//home/grawity/src/geocommit/.git/hooks/post-rewrite'
Merge pull request #12 from grawity/fix-gitdir

Determine Git directory correctly