How to push specific branch to remote with go-git

What is the canonical way to push specific single local branch to specific remote with go-git?

I have a local repository checked out and opened with go-git

repo, err := git.PlainOpen("my-repo")

The repo has default origin remote.

I'm trying to sync contents of this repository to another remote mirror, so I'm adding the remote

repo.CreateRemote(&config.RemoteConfig{
				Name: "mirror",
				URLs: []string{"git@github.com:foo/mirror.git"},
			})

First I fetch repo contents from origin

err = remote.Fetch(&git.FetchOptions{
				RemoteName: "origin",
				Tags:       git.AllTags,
			})

... and do a discovery of all branches and tags of interest with remote.List()

Final step is pushing the branches to mirror, while rewriting branch names based on a map. E.g. refs/remotes/origin/master checked out as refs/heads/master should be pushed to mirror remote as main. Therefore I'm iterating over the branches and trying to push them one by one:

refSpec := config.RefSpec(fmt.Sprintf(
				"+%s:refs/remotes/mirror/%s",
				localBranch.Name().String(),
                // map branch names, e.g. master -> main
				mapBranch(remoteBranch.Name().Short()),
			))
err = repo.Push(&git.PushOptions{
				RemoteName: "mirror",
				Force:      true,
				RefSpecs:   []config.RefSpec{refSpec},
				Atomic:     true,
			})

But this results in git.NoErrAlreadyUpToDate and there's nothing happening on the mirror remote.

The refSpec, when pushing single branch to a remote, should NOT be in format +refs/heads/localBranchName:refs/remotes/remoteName/remoteBranchName as indicate e.g. here:

// RefSpec is a mapping from local branches to remote references.
...
// eg.: "+refs/heads/*:refs/remotes/origin/*"
//
// https://git-scm.com/book/en/v2/Git-Internals-The-Refspec
type RefSpec string

but as

"+refs/heads/localBranchName:refs/heads/remoteBranchName"

instead. See example:

    refSpecStr := fmt.Sprintf(
	    "+%s:refs/heads/%s",
		localBranch.Name().String(),
		mapBranch(remoteBranch.Name().Short()),
	)
	refSpec := config.RefSpec(refSpecStr)
	log.Infof("Pushing %s", refSpec)
	err = repo.Push(&git.PushOptions{
		RemoteName: "mirror",
		Force:      true,
		RefSpecs:   []config.RefSpec{refSpec},
		Atomic:     true,
	})