FR: add GIT_CLONE_EXTRA_FLAGS

Description

Please add GIT_CLONE_EXTRA_FLAGS.

With native git clone now being supported (FF_USE_GIT_NATIVE_CLONE), it would be great to pass additional git arguments to it, as it is possible with GIT_FETCH_EXTRA_FLAGS

Our use-case: our runners have a local (rather fresh) copy of our git repos. To speed up cloning, we would like to pass --reference to git clone.