Speed up setup_local_dev.sh Drupal core clone

From what I understand, a shallow clone will remove the ability to do git bisect on Drupal core.

That can be a problem, since there have been instances before when an upstream change in core broke us, and we needed to bisect core to track it down.

I support changing to a shallow clone by default, but I'd like to see the following:

• We should document that this is a shallow clone, and the effect on git bisect (commit blocker)
• We should be able to override this behavior with an environment variable or something (commit blocker if it's easy, otherwise let's do this in a follow-up)
• If this is even possible, we should document how to convert the shallow clone to a deep clone (very nice to have)

Code and documentation look good to me. I don't have time to manually test it, so if @rahul_ could take that on (run the script, confirm it's fast, then make sure you can convert it to a deep clone using the commands in DEVELOPING.md), I think this is RTBC from me.

Merged into 8.x-2.x. Thanks!