Support GitHub default branches

[mrshu]

While pulldozer expects the default branch of the repos it clones to be master, GitHub now uses main as the default branch. As this value can be different per-repository, the proper fix would probably involve obtaining the default_branch value from the GitHub API.

Happy to help with putting a PR around that together -- I just wanted to first discuss what would be the best way of doing that withing the current setup.

Thanks!

cc @mayhewsw

[artnc]

Thanks for reporting! I haven't tested yet but this should do it. I'm avoiding the GitHub API if possible because it's subject to rate-limiting.