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;
 }