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.
Perhaps this should be a yaml file too rather than parameters?