> Passing it down as an arg is indeed problematic.
> Maybe we should just add yet-another buffer-<foo>-filename and change
> the corresponding places to so that instead of consulting
>
>     buffer-file-name
>
> they consult
>
>     (or buffer-file-name buffer-<foo>-filename)
>
> Maybe this could also be used for things like `vc-revision-other-window`.

This requires more testing, but at least such patch
basically would look like this: