Implement diff_build_graphs.sh

It runs multiproduct_kati against the pre-changed and post-changed
code and confirms that the ninja files remain unchanged

Test: ./diff_build_graphs.sh 'build/blueprint:work^' 'build/blueprint:work'
Bug: 67478260

Change-Id: Ib43f54428b7c5abbe7706f464ee16dc8d0ef6f76