COMP: Suggest "insteadof" git config only for ITKv3.

Since ITKv4 doesn't use submodule (see fab42d0ef) it only applies to ITKv3.

Fixes #2751

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22375 3bd1e089-480b-0410-8dfb-8563597acbee