Feature: Default branch on git repos

I'm working with a git repository in keybase which has a default branch that is not `master`. It would be handy if I could specify the default branch for the repo with the keybase command, like: `keybase git set-default ` or something. Similar to the way default branches work in GitHub: https://help.github.com/en/github/administering-a-repository/setting-the-default-branch

Start by tracing how the keybase command handles git repositories and where repository settings are stored. Review the proposed `keybase git set-default ` interface and the linked GitHub default-branch behavior. Done means a repository can specify and use a non-master default branch through the command.

Written by the indexing model from the issue text.