Automatically wrap long Git commit messages in Vim

Add this to your .vimrc:

au FileType gitcommit setlocal tw=72

While the other answers solve this problem perfectly well, I highly recommend you install Tim Pope's fugitive.vim.

fugitive.vim is an excellent plugin that brings Git functionality to Vim. It has a whole host of features that aren't relevant to your question, but are worth checking out. However, it can also help you remember to keep your commit messages the proper length:

Add this to your .vimrc:

filetype indent plugin on

From here.

2018 Update - Update vim

If you update vim, it will automatically highlight the first 50 characters of your title and wrap lines at 72 characters. It knows that you're editing a git commit file.

Mac users can use Homebrew:

brew install vim

If you still aren't seeing the syntax highlighting, make sure you have it enabled:

You need to have following settings in .vimrc file as per arch linux documentation

filetype plugin on
syntax on

In addition to other answers, use gqip to reformat a paragraph while editing.

See Vim Tips Wiki: Automatic formatting of paragraphs