sapling: add module

Adds a program module for [Sapling](https://sapling-scm.com/).

Since Sapling itself is very similar in nature to Mercurial,
`modules/programs/mercurial.nix` was copied to make this module with
the ignore pieces removed (Sapling respects gitignore).

modules/programs/sapling.nix

@@ -0,0 +1,83 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.sapling;
+
+  iniFormat = pkgs.formats.ini { };
+
+in {
+  meta.maintainers = [ maintainers.pbar ];
+
+  options = {
+    programs.sapling = {
+      enable = mkEnableOption "Sapling";
+
+      package = mkPackageOption pkgs "sapling" { };
+
+      userName = mkOption {
+        type = types.str;
+        description = "Default user name to use.";
+      };
+
+      userEmail = mkOption {
+        type = types.str;
+        description = "Default user email to use.";
+      };
+
+      aliases = mkOption {
+        type = types.attrsOf types.str;
+        default = { };
+        description = "Sapling aliases to define.";
+      };
+
+      extraConfig = mkOption {
+        type = types.attrsOf types.anything;
+        default = { };
+        description = "Additional configuration to add.";
+      };
+
+      iniContent = mkOption {
+        type = iniFormat.type;
+        internal = true;
+      };
+    };
+  };
+
+  config = mkIf cfg.enable (mkMerge [
+    {
+      home.packages = [ cfg.package ];
+
+      programs.sapling.iniContent.ui = {
+        username = cfg.userName + " <" + cfg.userEmail + ">";
+      };
+    }
+
+    (mkIf (!pkgs.stdenv.isDarwin) {
+      xdg.configFile."sapling/sapling.conf".source =
+        iniFormat.generate "sapling.conf" cfg.iniContent;
+    })
+    (mkIf (pkgs.stdenv.isDarwin) {
+      home.file."Library/Preferences/sapling/sapling.conf".source =
+        iniFormat.generate "sapling.conf" cfg.iniContent;
+    })
+
+    (mkIf (cfg.aliases != { }) {
+      programs.sapling.iniContent.alias = cfg.aliases;
+    })
+
+    (mkIf (lib.isAttrs cfg.extraConfig) {
+      programs.sapling.iniContent = cfg.extraConfig;
+    })
+
+    (mkIf (lib.isString cfg.extraConfig && !pkgs.stdenv.isDarwin) {
+      xdg.configFile."sapling/sapling.conf".text = cfg.extraConfig;
+    })
+    (mkIf (lib.isString cfg.extraConfig && pkgs.stdenv.isDarwin) {
+      home.file."Library/Preferences/sapling/sapling.conf".text =
+        cfg.extraConfig;
+    })
+  ]);
+}