Video description, where I borrow @holman's demo of the tribal knowledge you currently need to have to use this great feature, while mentioning how you use this user script after installing it.
♡Copyheart 2012, Johan Sundström. Copying is an act of love. Please share and use however you like!
NB: This user script doesn't rely on jQuery (so it'll happily run in Chrome's Isolated Worlds user script sand box, besides in Greasemonkey, where the site's version is around, or dotjs, where it's around all the time). If you are github and don't need to care about that, or want the shorter one, grab https://github.com/johan/user.js/blob/master/github.com/line-permalinks.user.js#L9-44 instead.