How do I override the .gitconfig [merge] ff=only setting in VS Code?

Question

I prefer to have git do only fast-forward merges automatically, so I have included a

[merge]
    ff = only

stanza in my .gitconfig file. When I do need to do a more complex merge, I want to do use VS Code as a three-way merge tool. Alas, when I try to do that, the ff = only setting prevents the merge from running in VS Code.

I'm hoping that there is a better option than editing the .gitconfig file every time I want to do a non-FF merge in VS Code. There is a way around this in PyCharm.