Enable use of local reference directories

*If* you already have on-disk local copies of the repos you're cloning,
using them as a reference can be a significant time and bandwidth
saving.

If you don't already have them, then *shrug* you're gonna have to use
the network. If your local repo is stale, I'm told git is Teh Clever and
will pull the missing refs from the network.
Apparently.