pkgs: add diff-flake

This is a nice helper to know what has changed after bumping a flake's
inputs.

pkgs/diff-flake/default.nix

@@ -0,0 +1,35 @@
+{ coreutils, git, gnused, lib, shellcheck, stdenvNoCC }:
+stdenvNoCC.mkDerivation {
+  pname = "diff-flake";
+  version = "0.1.0";
+
+  src = ./diff-flake;
+
+  phases = [ "buildPhase" "installPhase" ];
+
+  buildInputs = [
+    shellcheck
+  ];
+
+  buildPhase = ''
+    shellcheck $src
+  '';
+
+  installPhase = ''
+    mkdir -p $out/bin
+    cp $src $out/bin/diff-flake
+    substituteAllInPlace $out/bin/diff-flake
+    patchShebangs $out/bin/diff-flake
+  '';
+
+  cat = "${coreutils}/bin/cat";
+  git = "${git}/bin/git";
+  sed = "${gnused}/bin/sed";
+
+  meta = with lib; {
+    description = "Nix flake helper to visualize changes in closures";
+    homepage = "https://gitea.belanyi.fr/ambroisie/nix-config";
+    license = with licenses; [ mit ];
+    platforms = platforms.unix;
+  };
+}