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).

tests/modules/programs/sapling/default.nix

@@ -0,0 +1,4 @@
+{
+  sapling-basic = ./sapling-basic.nix;
+  sapling-most = ./sapling-most.nix;
+}

tests/modules/programs/sapling/sapling-basic.nix

@@ -0,0 +1,26 @@
+{ pkgs, ... }:
+
+{
+  programs.sapling = {
+    enable = true;
+    userName = "John Doe";
+    userEmail = "johndoe@example.com";
+  };
+
+  test.stubs.sapling = { };
+
+  nmt.script = let
+    configfile = if pkgs.stdenv.isDarwin then
+      "Library/Preferences/sapling/sapling.conf"
+    else
+      ".config/sapling/sapling.conf";
+
+    expected = builtins.toFile "sapling.conf" ''
+      [ui]
+      username=John Doe <johndoe@example.com>
+    '';
+  in ''
+    assertFileExists home-files/${configfile}
+    assertFileContent home-files/${configfile} ${expected}
+  '';
+}

tests/modules/programs/sapling/sapling-most.nix

@@ -0,0 +1,48 @@
+{ pkgs, ... }:
+
+{
+  programs.sapling = {
+    enable = true;
+    userName = "John Doe";
+    userEmail = "johndoe@example.com";
+    aliases = {
+      cm = "commit";
+      d = "diff --exclude=*.lock";
+      s = "status";
+      view = "!$HG config paths.default | xargs open";
+    };
+    extraConfig = {
+      pager.pager = "delta";
+      gpg.key = "not-an-actual-key";
+    };
+  };
+
+  test.stubs.sapling = { };
+
+  nmt.script = let
+    configfile = if pkgs.stdenv.isDarwin then
+      "Library/Preferences/sapling/sapling.conf"
+    else
+      ".config/sapling/sapling.conf";
+
+    expected = builtins.toFile "sapling.conf" ''
+      [alias]
+      cm=commit
+      d=diff --exclude=*.lock
+      s=status
+      view=!$HG config paths.default | xargs open
+
+      [gpg]
+      key=not-an-actual-key
+
+      [pager]
+      pager=delta
+
+      [ui]
+      username=John Doe <johndoe@example.com>
+    '';
+  in ''
+    assertFileExists home-files/${configfile}
+    assertFileContent home-files/${configfile} ${expected}
+  '';
+}