git: add option to use riff as diff tool (#5748)

https://github.com/walles/riff

modules/programs/git.nix

@@ -411,6 +411,29 @@ in {
           '';
         };
       };
+
+      riff = {
+        enable = mkEnableOption "" // {
+          description = ''
+            Enable the <command>riff</command> diff highlighter.
+            See <link xlink:href="https://github.com/walles/riff" />.
+          '';
+        };
+
+        package = mkPackageOption pkgs "riffdiff" { };
+
+        commandLineOptions = mkOption {
+          type = types.listOf types.str;
+          default = [ ];
+          example = literalExpression ''[ "--no-adds-only-special" ]'';
+          apply = concatStringsSep " ";
+          description = ''
+            Command line arguments to include in the <command>RIFF</command> environment variable.
+
+            Run <command>riff --help</command> for a full list of options
+          '';
+        };
+      };
     };
   };
 
@@ -434,6 +457,7 @@ in {
             cfg.diff-so-fancy.enable
             cfg.difftastic.enable
             cfg.diff-highlight.enable
+            cfg.riff.enable
           ];
         in count id enabled <= 1;
         message =
@@ -678,5 +702,25 @@ in {
           };
         };
     })
+
+    (let riffExe = baseNameOf (getExe cfg.riff.package);
+    in mkIf cfg.riff.enable {
+      home.packages = [ cfg.riff.package ];
+
+      # https://github.com/walles/riff/blob/b17e6f17ce807c8652bc59cd46758661d23ce358/README.md#usage
+      programs.git.iniContent = {
+        pager = {
+          diff = riffExe;
+          log = riffExe;
+          show = riffExe;
+        };
+
+        interactive.diffFilter = "${riffExe} --color=on";
+      };
+    })
+
+    (mkIf (cfg.riff.enable && cfg.riff.commandLineOptions != "") {
+      home.sessionVariables.RIFF = cfg.riff.commandLineOptions;
+    })
   ]);
 }