dev/release: reference local clone when cloning
Created by: keegancsmith
This drastically speeds up operations for me. Cloning the Sourcegraph repo takes a minute or two for me thanks to Cape Town being a bit too far from GitHub's data centers.
Reference will avoid fetching objects that are already in the local clone. We use the -if-able variant just in case things go wrong, then we still proceed with the normal clone (if not a little slower).
Test Plan: dry run without main branch check then ran the release command. Total time was 28s instead of 1m38s.