Git difftool -y --merge-base B -- compares with B instead of fork point

[pavoljuhas]

I often work on a feature-branch which gets behind a fast-moving main branch.
I would like to see changes in the feature-branch w/r to its fork-point from main.
In a stock git I can do (assuming feature-branch is checked out)

git diff --merge-base main

I tried to accomplish something similar with vim-fugitive using

Git difftool -y --merge-base main

This correctly opened new tabs for files changed in the feature-branch (skipping later changes in main),
however the files were compared to the main rather than to the fork point from main.

Would it be possible to get this fixed?

I see this behavior in the latest fugitive version at 320b18f.