Bounce explicit LocalCommits render in refreshBranches to UI thread

refreshBranches runs on a worker goroutine and re-renders the commits view
directly to refresh the branch-head visualization. As with refreshView, this
currently only flushes because HandleRender ends with self.c.Render(). Wrap the
explicit HandleRender call (along with the LocalCommitsMutex pair around it) in
OnUIThread so it keeps working once Render() is removed.