Change default branch name to main

[justmoon]

Changed the default branch name from master to main.

See: https://www.hanselman.com/blog/EasilyRenameYourGitDefaultBranchFromMasterToMain.aspx

@rvagg - Please update your local working copy:

$ git checkout master
$ git branch -m master main
$ git fetch
$ git branch --unset-upstream
$ git branch -u origin/main
$ git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main