[RFC] .gitmodules: Make Externals/Qt clone shallow

[spycrab]

Instead of cloning the entire history of the submodule, only fetch the latest commit. This saves about 300M of disk space currently and will only save more in the future.

Caveats:

• A shallow clone might be slower under some circumstances such as a very fast internet connection (Try this test)
• If you want to change anything in the Externals you will have to unshallow the clone (realistically only about 5 people ever needed to do this)

[leoetlino]

Sounds good to me

[leoetlino]

Even with a 100Mbps connection, this saved ~15s of cloning time in addition to the substantial disk space usage reduction. I'd expect the time savings to be greater for most people considering that the average Internet connection speed is lower than 100Mbps...

[stenzek]

Also sounds good to me.