Patch Set 2: I would prefer that you didn't submit this

Instead of refusing to check out a project that has a .gitmodules file, I would prefer to fix the code that is trying to process a GITLINK entry as though it were a blob.

FWIW, even C Git ran into a ton of bugs when GITLINK was added to the core code.  It took them a while to shake out all of the code paths that cared.