fix: use correct diff layout

[zegervdv]

When running DiffviewOpen on a merge conflict I get the following error:

The coroutine failed with this message: ...nvim/lua/diffview/scene/views/standard/s
tandard_view.lua:143: attempt to index a nil value                                                                                                                                            
stack traceback:                                                                                                                                                                              
        [C]: in function 'error'                                                                                                                                                              
        ...share/nvim/lazy/plenary.nvim/lua/plenary/async/async.lua:18: in function 'callback_or_next'                                                                                        
        ...share/nvim/lazy/plenary.nvim/lua/plenary/async/async.lua:45: in function <...share/nvim/lazy/plenary.nvim/lua/plenary/async/async.lua:44> 

I've bisected the repo and found 5fc2b59e9668c06588e9f0cacb79d459067ae273 to have introduced this.

Seems like the layout_key can be nil when opening the view.

[sindrets]

Seems like the actual root of the problem here is that I'm importing the wrong thing here:

https://github.com/sindrets/diffview.nvim/blob/3733cdad0ec88e6f5e6a07d6e9cf488c87a82d38/lua/diffview/scene/views/standard/standard_view.lua#L6

"...layouts.diff_3" should be "...layouts.diff_4"

So we've never actually been checking whether the layout was a Diff4 here.

[zegervdv]

updated