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; {
@@ -34,35 +37,43 @@ in
 
     home.packages = [ cfg.package ];
 
+    xdg.configFile."delta/config".source = configFormat.generate "delta-config" {
+      delta = {
+        features = "diff-highlight decorations";
+
+        # Less jarring style for `diff-highlight` emulation
+        diff-highlight = {
+          minus-style = "red";
+          minus-non-emph-style = "red";
+          minus-emph-style = "bold red 52";
+
+          plus-style = "green";
+          plus-non-emph-style = "green";
+          plus-emph-style = "bold green 22";
+
+          whitespace-error-style = "reverse red";
+        };
+
+        # Personal preference for easier reading
+        decorations = {
+          commit-style = "raw"; # Do not recolor meta information
+          keep-plus-minus-markers = true;
+          paging = "always";
+        };
+      };
+    };
+
     programs.git = lib.mkIf cfg.git.enable {
       delta = {
         enable = true;
         inherit (cfg) package;
-
-        options = {
-          features = "diff-highlight decorations";
-
-          # Less jarring style for `diff-highlight` emulation
-          diff-highlight = {
-            minus-style = "red";
-            minus-non-emph-style = "red";
-            minus-emph-style = "bold red 52";
-
-            plus-style = "green";
-            plus-non-emph-style = "green";
-            plus-emph-style = "bold green 22";
-
-            whitespace-error-style = "reverse red";
-          };
-
-          # Personal preference for easier reading
-          decorations = {
-            commit-style = "raw"; # Do not recolor meta information
-            keep-plus-minus-markers = true;
-            paging = "always";
-          };
-        };
       };
+
+      includes = [
+        {
+          path = configPath;
+        }
+      ];
     };
   };
 }