vertical dif split of a file between different branches

[miEsMar]

I am trying to open a vertical diff split view for a file between HEAD and branch-name.
Basically:

git diff HEAD branch-name --  file.txt

I've tried with:

:Gvdiffsplit :% <branch-name>:%

but while it correctly picks the ":%" argument, it shows an empty file content on the right.

How to achieve this in fugitive?

Thanks