home: delta: use stand-alone configuration file

Ideally, I'd like for `delta` to just read a configuration file at
`$XDG_CONFIG_HOME/delta/config` by default, but upstream seems somewhat
reticent to the idea :-/.

So instead, let's keep relying on `git` being enabled, but rather than
inlining the configuration, let's store it where I think it should
belong and include it into `gitconfig`.

modules/home/delta/default.nix

@@ -1,6 +1,9 @@
 { config, pkgs, lib, ... }:
 let
   cfg = config.my.home.delta;
+
+  configFormat = pkgs.formats.gitIni { };
+  configPath = "${config.xdg.configHome}/delta/config";
 in
 {
   options.my.home.delta = with lib; {
@@ -34,12 +37,8 @@ in
 
     home.packages = [ cfg.package ];
 
-    programs.git = lib.mkIf cfg.git.enable {
+    xdg.configFile."delta/config".source = configFormat.generate "delta-config" {
       delta = {
-        enable = true;
-        inherit (cfg) package;
-
-        options = {
         features = "diff-highlight decorations";
 
         # Less jarring style for `diff-highlight` emulation
@@ -63,6 +62,18 @@ in
         };
       };
     };
+
+    programs.git = lib.mkIf cfg.git.enable {
+      delta = {
+        enable = true;
+        inherit (cfg) package;
+      };
+
+      includes = [
+        {
+          path = configPath;
+        }
+      ];
     };
   };
 }