Add --force option to fetch_sources.py
Add --force option to fetch_sources.py

This passes --force to git fetch and checkout.

Change-Id: I56a7ccafefb2eaac69d2253516308daa8e8808cf
(cherry picked from commit 2feac9e222d15c94adf3d39e61878e489a0f0aea)

KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS