Possibility to use git difftool

[m0tay]

Is your feature request related to a problem? Please describe.
I want to use my configured git difftool (nvimdiff).

Describe the solution you'd like
I would love the possibility to invoke git difftool (or nvimdiff, even vimdiff for that matter) directly from the Neogit.

Describe alternatives you've considered
Using diffview.nvim, but it feels like "overkill" for me, also one more plugin...

Manually running commands...

Native Vim diffs: like :vert diffsplit manually, but this doesn't automatically grab the correct HEAD/Index versions like difftool does...

Additional context
Neogit has a great staging view. I want to use my own diff tool. No extra plugins/dependencies needed.

[winston0410]

It would be great if the new neovim's default difftool can be supported.

[m0tay]

@winston0410 yeah, it'd be.