You're talking about the title of the commit message. Yes, the title is supposed to be under 50 characters. Now add a blank line after it and you can write all you want (line wrap at 72 chars).

If you use markdown Syntax in your commit body, github and other tools will gladly render it correctly. You can grep for commit messages with "git log --grep <pattern>" and it will also search commit bodies as well. It's beautiful!

Adding on to sibling comment, on GitHub the additional lines they're talking about go in the box before the commit title.