Prevent html injection

Commits page renders commit description with single_format method which allows html tags. So commit message with html tags brokers Commits page. See screenshot

![Screenshot 2014-07-10 11.16.40](https://dev.gitlab.org/uploads/gitlab/gitlabhq/6606e1bac0/Screenshot_2014-07-10_11.16.40.png)

See merge request !959