repo: Upgrade: use branch or commit

While doing newt upgrade the script tries to checkout
to 'master' while there is 'main' instead. Now it is possible
to use the 'branch: 'main'' in repository.yml file,
and it doesn't only allow to fetch properly, but also to checkout.

example from repository.yml:

repository.xyz:
    type: git
    vers: 0.0.0
    branch: 'main'
    url: 'git@github.com:xyz.git'

newt/repo/repo.go

@@ -323,6 +323,10 @@ func (r *Repo) Upgrade(ver newtutil.RepoVersion) error {
 		return err
 	}
 
+	if branch := r.downloader.MainBranch(); branch != "" {
+		commit = branch
+	}
+
 	if err := r.updateRepo(commit); err != nil {
 		return err
 	}