Update init-repository.py to allow using upstream chromium repository.

The regular workflow now is just to clone the qtwebengine repository
and then execute the following commands.
  git submodule init && git submodule update --recursive && qmake && make
This will also clone a submodule called 3rdparty which contains the
chromium and ninja sources without pulling in further submodules
and without a complete history.

Developers that do want to have a complete chromium checkout instead
should not use the above command sequence.
Instead they should just clone qtwebengine, execute the
init-repository.py script, run qmake and make.
The init-repository.py script will then checkout the complete
chromium sources into a subdirectory called 3rdparty_upstream.
The location of these sources will be picked up by qmake automatically.

Change-Id: I0fa4f1d554bdca2e852b6a97aa2e5462d90d8664
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
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>