Use git to checkout files instead of using the download url from github

[drdanz]

Instead of downloading the files from raw.githubusercontent.com, it might be possible to checkout only one file, see https://stackoverflow.com/questions/2466735/checkout-only-one-file-from-git

Requires git >= 1.7.9.5 and tar, so while the first might ok, the latter might be a problem on windows, we should test if it works

This could eventually also be a feature in IncludeUrl

[drdanz]

Note: Instead of tar, we could use cmake -E tar