"Fork his repo" is a reference to git. There is nothing sexual about it.

Here's a repo for my hackerspace's website. At the top right of the page is the word "fork".

https://github.com/heatsynclabs/new-hsl

It's worth pointing out that Adria isn't a developer, and might not have realized that this wasn't a sex joke, he was talking about flattering somebody by forking their git repo.