reproducible FreeBSD: Switch master to main and use new repo name

During the SVN-to-Git migration, which resulted in new hashes compared
with the old git-svn repo, the branch name for -CURRENT was changed from
master to main, and the mirror was renamed to freebsd-src on GitHub, to
match the repo on the authoritative (c)git(repo).freebsd.org being
called src.git.