[vim] Add `fugitive` plugin

Ref: https://github.com/tpope/vim-fugitive

See also:

 * http://vimcasts.org/episodes/fugitive-vim---a-complement-to-command-line-git/
 * http://vimcasts.org/episodes/fugitive-vim-working-with-the-git-index/
 * http://vimcasts.org/episodes/fugitive-vim-resolving-merge-conflicts-with-vimdiff/
 * http://vimcasts.org/episodes/fugitive-vim-browsing-the-git-object-database/
 * http://vimcasts.org/episodes/fugitive-vim-exploring-the-history-of-a-git-repository/