github private with git credentials

I have a private package kept with github, using their fee-enabled **private** setting. To use it locally, first I clone it and thereafter I use checkout. Checkout prompts me for the username and password. My username and password are stored using git's credentials manager.

Pkg3's clone and checkout should find them.

Start by tracing Pkg3's clone and checkout paths and how they invoke Git, then inspect the interaction with Git's credentials manager. Done means private GitHub packages use stored credentials without prompting during both clone and checkout.

Written by the indexing model from the issue text.