You’ll need to be using a build of UE4 from Github. The binary build from the launcher does not allow you to edit the source code.
The reason the commit link was not working for you is because you most likely either haven’t logged into your linked Github account, or you have never linked your Unreal Engine and Github accounts. Here’s the wiki for how to do this: A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums