Wrap at 72 characters for git commit messages.

The body of a git commit message is conventionally ([1], [2]) wrapped at
72 characters. This commit adjust .vimrc to automatically wrap the body
of git commit messages at 72 characters.

[1]: http://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message
[2]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html