overlays: add 'gruvbox-nvim-old-diff'

I dislike the new style of diff [1].

Thankfully somebody wrote a patch to configure it [2] (though not
completely to my liking, so the local patch here is a bit different).
I used it as a basis, but made it always revert, rather than
configurable.

[1]: https://github.com/ellisonleao/gruvbox.nvim/issues/290
[2]: https://github.com/ellisonleao/gruvbox.nvim/pull/291

overlays/gruvbox-nvim-old-diff/generated.nix

@@ -0,0 +1,24 @@
+{ vimUtils, fetchFromGitHub }:
+
+_final: _prev: {
+  gruvbox-nvim = vimUtils.buildVimPlugin {
+    pname = "gruvbox.nvim";
+    version = "2023-10-07";
+
+    src = fetchFromGitHub {
+      owner = "ellisonleao";
+      repo = "gruvbox.nvim";
+      rev = "477c62493c82684ed510c4f70eaf83802e398898";
+      sha256 = "0250c24c6n6yri48l288irdawhqs16qna3y74rdkgjd2jvh66vdm";
+    };
+
+    patches = [
+      # Inspired by https://github.com/ellisonleao/gruvbox.nvim/pull/291
+      ./old-colours.patch
+    ];
+
+    meta = {
+      homepage = "https://github.com/ellisonleao/gruvbox.nvim/";
+    };
+  };
+}