fzf.vim - How to search git branch command with git log preview?

Question

I'm trying to create neovim fzf command FzfGBranches to use git branch -a as query, and use git log --oneline as fzf preview:

command! -bang -nargs=0 FzfGBranches
            \ call fzf#vim#grep(
            \ "git branch -a", 1,
            \ fzf#vim#with_preview({
            \   'options': [
            \       '--prompt', '*Branches> ',
            \       '--bind', 'ctrl-d:page-down,ctrl-u:page-up',
            \   ],
            \   'placeholder': "echo {} | rev | cut -d'*' -f1 | rev | git log --oneline --graph --date=short --color=always --pretty=\"format:%C(auto)%cd %h%d %s\"",
            \ }), <bang>0)

When I call FzfGBranches command, it show up and seems good, but when I press up/down keys, the preview (on the right side) does not refresh. See below screenshot:

How should I specify the --preview option in fzf.vim?