git: add basic support for delta

PR #1198

modules/misc/news.nix

@@ -1503,6 +1503,16 @@ in
           A new module is available: 'programs.aria2'
         '';
       }
+
+      {
+        time = "2020-05-04T21:19:43+00:00";
+        condition = config.programs.git.enable;
+        message = ''
+          The Git module now supports the 'delta' syntax highlighter.
+
+          It can be enabled through the option 'programs.git.delta.enable'.
+        '';
+      }
     ];
   };
 }

modules/programs/git.nix

@@ -215,6 +215,24 @@ in {
           '';
         };
       };
+
+      delta = {
+        enable = mkEnableOption "" // {
+          description = ''
+            Whether to enable the <command>delta</command> syntax highlighter.
+            See <link xlink:href="https://github.com/dandavison/delta" />.
+          '';
+        };
+
+        options = mkOption {
+          type = types.listOf types.str;
+          default = [ ];
+          example = [ "--dark" ];
+          description = ''
+            Extra command line options given to delta.
+          '';
+        };
+      };
     };
   };
 
@@ -309,5 +327,16 @@ in {
             ([ "git-lfs" "smudge" ] ++ skipArg ++ [ "--" "%f" ]);
         };
     })
+
+    (mkIf cfg.delta.enable {
+      programs.git.iniContent = let
+        deltaArgs = [ "${pkgs.gitAndTools.delta}/bin/delta" ]
+          ++ cfg.delta.options;
+      in {
+        core.pager = concatStringsSep " " deltaArgs;
+        interactive.diffFilter =
+          concatStringsSep " " (deltaArgs ++ [ "--color-only" ]);
+      };
+    })
   ]);
 }

tests/modules/programs/git/git-expected.conf

@@ -6,6 +6,9 @@
 [commit]
 	gpgSign = true
 
+[core]
+	pager = "@deltaCommand@ --dark"
+
 [extra]
 	boolean = true
 	integer = 38
@@ -28,6 +31,9 @@
 [gpg]
 	program = "path-to-gpg"
 
+[interactive]
+	diffFilter = "@deltaCommand@ --dark --color-only"
+
 [user]
 	email = "user@example.org"
 	name = "John Doe"

tests/modules/programs/git/git.nix

@@ -15,6 +15,8 @@ let
     pkgs.substituteAll {
       src = path;
 
+      deltaCommand = "${pkgs.gitAndTools.delta}/bin/delta";
+
       git_include_path = pkgs.writeText "contents"
         (builtins.readFile ./git-expected-include.conf);
     };
@@ -56,6 +58,10 @@ in {
         userEmail = "user@example.org";
         userName = "John Doe";
         lfs.enable = true;
+        delta = {
+          enable = true;
+          options = [ "--dark" ];
+        };
       }
 
       {