Update Git Cheat Sheet PDF URL in Quick Reference links
Git for Windows now releases on GitHub

Change the 'download' task to look at the GitHub releases. The
URL-building is suboptimal, but the GitHub API does not provide a way to
get a link that one can pass to a browser, so we build the one from the
site by hand.
Redirect download links away from Google Code

Google Code has shut off its download service. Git release tarballs are
now only on kernel.org and Git for Windows releases are on GitHub.

This fixes #384.
Merge pull request #394 from carlosmn/msysgit-download

Git for Windows now releases on GitHub
Merge pull request #395 from carlosmn/source-downloads

Redirect download links away from Google Code
Merge pull request #391 from jordanmccullough/cheat-sheet-reference

Update Git Cheat Sheet PDF URL in Quick Reference links