Blender Git Commit Log

Use Git submodule tracking feature

This way we can point submodules to different branches.

There are two side-effects to this:

- Git 1.8.2 becomes the minimal required version now
to support this feature.

- Not sure how doing local changes in submodules followed
by `make update` will behave. We don't use explicit rebase
now.

Perhaps this is not so bad, since it was already quite
dangerous thing to do.