github-commit-link: Support user/repo@SHA and shorten response

Details are in the commit messages themselves, but the support for
repo@<SHA> and user/repo@<SHA> is the most important to me.  Being
able to reference commits made in arbitrary GitHub projects makes it
easy to explain my motivation:

    Jack: I don't understand why we need 123abcd
    Jill: Because joe/widgets@9a8b7c6 broke compatibility with their old API