How can I get fzf to preview a git diff when there's 2 directories being diff'd?

[nickjj]

I am trying to make a general purpose diffing script. git diff lets you diff 2 non-git directories.

I almost have it with this, but the file contents in the preview never updates when I select a new item:

git diff --name-only dir_a dir_b | fzf --ansi --preview "git diff --color=always dir_a dir_b"

I've seen other examples where you do git diff --color=always dir_a dir_b -- {-1} but when I do this I get an error in the preview window with fatal: bad revision dir_a. -- {-1} seems to only work when running git diff without more than 1 argument (such as when diffing something within a git repo, not 2 directories in different paths).