feature: Diff split like fugitive

[jcorbin]

Did you check the docs?

• [X] I have read all the docs

Is your feature request related to a problem? Please describe.

The one thing keeping me stuck on fugitive is its :GSplit feature, which allows me to never need to run git-add-p in practice.

Describe the solution you'd like

A simple diff split mode:

• the key feature that a plugin needs to provide, is the ability to directly edit a git stage blob
• from there it’s mostly a matter of regular vimdiff features and editing

Describe alternatives you've considered

Was hoping that diffview.nvim would work, allowing me to switch off fugitive to it plus a lua native porcelain like neogit (or fugit2…). But their focus on multi vcs abstraction, and fancy tree spanning changeset orientation doesn’t work well for me: https://github.com/sindrets/diffview.nvim/issues/471

Additional context

No response