How, exactly? I'm genuinely curious, I haven't seen any sign of "extension" like IE6 extensions to web standards

Github itself extends Git in a closed and proprietary way...

Github is the extend part or EEE, Git is the Standard, Github is the extension...

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Edit since I being prevented from responding I will edit this to address the idea that "Github is not an extension of git"

Yes, it is. While you can still use git with out Github, you could also use "the web" with out IE6, GitHub however had workflow and process improvements over standard git, just like Active X added workflow and process improvements over HTML Standard.

GitHub adds things like Issues, Pull Requests, "Social Coding, unified Login, Access Controls, Project Discovery and about 100+ other things on top of git that using git alone does not get you. This is text book example of EXTEND

it is insane to me how people do not see it.

Further to address "I have to be insane to believe GitHub was started by MS" well I never said that, Simply because Github was not created by MS, does not change the fact that their buying Github is part of the EEE model

GitHub is a set of UI tools built on top of a command line interface. It is not an extension to git. You can still use git normally. You can use other UI tools built on top of git. That, by definition, is not an extension.

And the logical leap required to think that somehow GitHub was built, years ago, as part of a strategy for Microsoft is bordering on insanity.