On October 1, 2020: newly-created repositories will default to main

https://github.com/github/renaming#on-october-1-2020-newly-created-repositories-will-default-to-main

GitHub will use main as the default branch soon. However, most of our scripts hardcode master as the default branch of the upstream.
We need to update them and allow user to specify the branch via git config --get git-extras.default-branch.

PR is welcome!