Added support for GitHub Gists (gist.github.com)

[tionis]

GitHub gists under gist.github.com are just git repos that share the same auth infrastructure as the normal github.com instance, so we can just reuse the existing github.com configuration there.

I tested it by pushing to GitHub gist (cloning is free for all when you know the URL, but pushing is limited to the owner) The implementation duplicates the magic numbers in the code, but they are right next to each other so it should still be easy to change if needed.

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