Initial support for checkout a tree url in addition to pull-request urls.

Getting used to use git co <pullrequest-url> and then see it fail when using a tree url (ttps://github.com/jbosstools/jbosstools-forge/tree/jbosstools-4.2.x) 
always annoyed me.

Here is the basic initial implementation of this.

I would like to write tests for it too, but the current one fails so I'm not sure how to
add tests for it knowing if I broke something. 

Any pointers would be appreciated on what to do about that.