Github's Wiki page shows contents with markdown-emulated page (e.g. https://github.com/github/gollum/wiki ), not the plain text of the .md file.
Where does Github provide us with the source of the .md file on its Wiki page?
Simple clone the wiki of that project:
git clone https://github.com/github/gollum.wiki.git
You will get .md files there.
C:\Users\VonC\prog\git\gollum.wiki>dir
29/01/2014 07:44 <REP> .
29/01/2014 07:44 <REP> ..
29/01/2014 07:44 0 .md
29/01/2014 07:44 2 hello.md
29/01/2014 07:44 11 649 Home.md