Configure simplified git mergetool

I still use neovim, but thanks to zdiff3 diffstyle there is no need for
a 3 way diff as UI.
I can use the final result as simple file instead, which I did for a
long time already.
But now I get rid of the unnecessary files and buffers.

home/programs/git.nix

@@ -135,11 +135,11 @@
     merge = {
       ff = true;
       conflictstyle = "zdiff3";
-      tool = "nvimdiff4";
+      tool = "nvim";
     };
 
-    "mergetool \"nvimdiff4\"" = {
-      cmd = "nvim -d $LOCAL $BASE $REMOTE $MERGED -c '$wincmd w' -c 'wincmd J' -c 'diffoff'";
+    "mergetool \"nvim\"" = {
+      cmd = "nvim $MERGED";
     };
   };
 }