.gitmodules: Add "branch = master" to all of them

Hopefully this makes it follow the master branch and I don't have to
create a commit here every time there's something new happening in
Git/* . Have my doubts, though.

97535974-f6e9-11e2-864d-001f3b596ec9
Nope, still have to commit changes in submodules

There has to better solutions for this. I just want it to track the
latest version on master, whatever commit that might be.

6759f6fe-f6eb-11e2-a236-fefdb24f8e10