To be clear, I think many developers are comfortable enough with the small handful of commands most regularly use. For the more advanced case of maintaining a fork of a high-velocity codebase like WebKit, it's likely you'll need a deeper understanding of remotes and how to manage complex rebases, especially in the presence of lots of conflicts. And possibly some fancier tools like git-subtree. In particular in WebKit my recollection was it was common to patch something locally and eventually take it upstream, but after upstream's requested modifications the patch would eventually come back around and conflict with itself.
