WIP: 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 :-/.

TODO:
* `git.enable = false` would break the standalone configuration
* using a wrapper which sets `--config` might be better than includes

modules/home/default.nix

@@ -8,6 +8,7 @@
     ./bluetooth
     ./calibre
     ./comma
+    ./delta
     ./dircolors
     ./direnv
     ./discord

modules/home/delta/default.nix

@@ -0,0 +1,72 @@
+{ 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; {
+    enable = my.mkDisableOption "delta configuration";
+
+    package = mkPackageOption pkgs "delta" { };
+
+    git = {
+      enable = my.mkDisableOption "git integration";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        # For its configuration
+        assertion = cfg.enable -> cfg.git.enable;
+        message = ''
+          `config.my.home.delta` must enable `config.my.home.delta.git` to be
+          properly configured.
+        '';
+      }
+    ];
+
+    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;
+      };
+
+      includes = [
+        {
+          path = configPath;
+        }
+      ];
+    };
+  };
+}

modules/home/git/default.nix

@@ -42,34 +42,6 @@ in
 
     lfs.enable = true;
 
-    delta = {
-      enable = true;
-
-      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";
-        };
-      };
-    };
-
     # There's more
     extraConfig = {
       # Makes it a bit more readable