git: configure patdiff through [patdiff] git section (#6978)

This adds the ability to enable the patdiff program as git differ.
Website of the project: https://opensource.janestreet.com/patdiff/

tests/modules/programs/git/default.nix

@@ -9,4 +9,5 @@
   git-without-signing = ./git-without-signing.nix;
   git-with-hooks = ./git-with-hooks.nix;
   git-with-maintenance = ./git-with-maintenance.nix;
+  git-patdiff = ./git-patdiff.nix;
 }

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

@@ -0,0 +1,19 @@
+[commit]
+	gpgSign = true
+
+[diff]
+	external = "@patdiff@/bin/patdiff-git-wrapper"
+
+[gpg]
+	format = "openpgp"
+
+[gpg "openpgp"]
+	program = "path-to-gpg"
+
+[tag]
+	gpgSign = true
+
+[user]
+	email = "user@example.org"
+	name = "John Doe"
+	signingKey = "00112233445566778899AABBCCDDEEFF"

tests/modules/programs/git/git-patdiff.nix

@@ -0,0 +1,20 @@
+{
+  programs.git = {
+    enable = true;
+    signing = {
+      signer = "path-to-gpg";
+      format = "openpgp";
+      key = "00112233445566778899AABBCCDDEEFF";
+      signByDefault = true;
+    };
+    userEmail = "user@example.org";
+    userName = "John Doe";
+
+    patdiff.enable = true;
+  };
+
+  nmt.script = ''
+    assertFileExists home-files/.config/git/config
+    assertFileContent home-files/.config/git/config ${./git-patdiff-expected.conf}
+  '';
+}