Because of this line:
public const string MarginName = "GitDiffMargin";
the bundled GitDiffMargin conflicts with the unbundled one (http://visualstudiogallery.msdn.microsoft.com/cf49cf30-2ca6-4ea0-b7cc-6a8e0dadc1a8
Specifically, although it is possible to disable the bundled diff margin, doing so disables the unbundled one as well.
I believe renaming the bundled MarginName from GitDiffMargin to GitSccDiffMargin, e.g., would remove the conflict.
This is desirable because the bundled one exhibits bugs that the unbundled one does not exhibit.