Actually, why isn't there a git extension for github tasks? Theres no reason why I shouldn't be able to do git send-github gitusername <branch> and have it pop up a commit editor to type the pull request text, etc.
But that's not an extension. It could easily be. The fact that it isn't, and Defunkt's suggestion to alias hub to git nicely illustrates the original author's point.
Its not an extension, but close enough. I cannot upvote you enough.