What does this red square icon mean in Visual Studio 2013 Solution Explorer?

ankhsvn, resharper, visual-studio-2013

Solution

It's likely from Subversion, saying that you have changes in the file

Problem

In the image below, there are red square icons next to items in the Solution Explorer. I can't figure out what they indicate. I'm using Resharper and AnkhSVN, so they may be from either of those two packages, or from Visual Studio itself. Does anyone have any idea what they mean?

Original source