Not sure about this: configure our submodules to be shallow

[kmod]

Fixes #75 issue 1

There are some drawbacks to this so I don't know if we should merge this, but it was easy to change so I'm putting it up.

With the change:

• the .git folder goes from 3.8GB to 3.2GB
• the total checkout goes from 6.1GB to 5.5GB
• the initial clone takes much longer. I didn't time it but I estimate it went from about 1 minute to 5 minutes. I think this is because of the extra load on the server, where shallow clones don't benefit from the same optimizations that a full clone receives.
• It'd be annoying to change the submodules, though we rarely do that

[undingen]

I don't know enough about git so really decide if this a good idea or not.
But in general I sounds like a good trade-off to me (less disk usage and network bandwidth).
How will the workflow for updating a submodule for us change? I guess we will have to do a full checkout but is something else necessary?

[kmod]

I think I'm -1 on this change mostly because of the clone-time increase and that it's only a 20% improvement in disk space, but I could be convinced

As for workflow, I think you can revert this commit and then when you update your submodules they will convert back to full clones, then you can update them, and then presumably people with shallow clones will get updated to a new shallow clone.