Use GIT_DEPTH when cloning submodules
What does this MR do?
This uses GIT_CLONE to determine the depth we should use for cloning submodules.
Why was this MR needed?
Cloning large submodules can take a long time. This reduces that time, since we don't grab the full history.
What's the best way to test this MR?
Run it and verify that submodule depths are set correctly? I don't really understand how testing works for this project. I did run tests, but they took a long time. There were a few failures, but I don't know if they were related to this MR.