Blender Git Loki

(See http://codereview.appspot.com/5431064/ for review of patch)

I've written a convenient function that returns the sibling of a node in the
red-black tree implementation originally implemented by Joshua Leung.

I want to use this get_sibling() function in the future to implement the missing
removal function of the red-black tree implementation.

For now the get_sibling() function just simplifies the get_uncle() function.

Just like the rest of the red-black tree implementation this diff is based on
Wikipedia: http://en.wikipedia.org/wiki/Red%E2%80%93black_tree#Removal