Add vim-git vim plugin

vim-git includes are syntax, indent, and filetype plugin files for git,
gitcommit, gitconfig, gitrebase, and gitsendemail.

GitHub repo: https://github.com/tpope/vim-git

Referenced in Tim Pope's excellent blog post on git commit messages:
http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html