Added gitref.org. I've found this site really useful :)

free-programming-books.md

@@ -413,6 +413,7 @@
 * [Git From The Bottom Up](http://ftp.newartisans.com/pub/git.from.bottom.up.pdf) (PDF)
 * [Git internals](https://github.com/pluralsight/git-internals-pdf/raw/master/drafts/peepcode-git.pdf)
 * [Git Magic](http://www-cs-students.stanford.edu/~blynn/gitmagic/)
+* [Git Reference](http://www.gitref.org)
 
 
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