Make sure submodule changes do not show up as a diff.

This is needed to prevent .gitmodules changes
from showing up when using the upstream repositories.
It will also prevent changes in submodules from showing
up and being added to a commit by accident.

Change-Id: Ia74f59ebce6d458b39c6ca2472aa6e623cd280fd
Reviewed-by: Andras Becsi <andras.becsi@digia.com>