diffoscope: Reduce closure size

This reduces diffoscope's closure size from 2470 MiB to 579 MiB by
leaving out some less crucial dependencies (like GHC and Free
Pascal). These can be re-enabled by turning on enableBloat.

pkgs/tools/misc/diffoscope/default.nix

@@ -1,6 +1,7 @@
-{ stdenv, fetchgit, pythonPackages, docutils
+{ lib, stdenv, fetchgit, pythonPackages, docutils
 , acl, binutils, bzip2, cbfstool, cdrkit, cpio, diffutils, e2fsprogs, file, fpc, gettext, ghc, gnupg1
 , gzip, jdk, libcaca, mono, pdftk, poppler_utils, rpm, sng, sqlite, squashfsTools, unzip, vim, xz
+, enableBloat ? false
 }:
 
 pythonPackages.buildPythonPackage rec {
@@ -22,9 +23,11 @@ pythonPackages.buildPythonPackage rec {
 
   # Still missing these tools: enjarify otool(maybe OS X only) showttf
   # Also these libraries: python3-guestfs
+  # FIXME: move xxd into a separate package so we don't have to pull in all of vim.
   propagatedBuildInputs = (with pythonPackages; [ debian libarchive-c python_magic tlsh ]) ++
-    [ acl binutils bzip2 cbfstool cdrkit cpio diffutils e2fsprogs file fpc gettext ghc gnupg1
-      gzip jdk libcaca mono pdftk poppler_utils rpm sng sqlite squashfsTools unzip vim xz ];
+    [ acl binutils bzip2 cbfstool cdrkit cpio diffutils e2fsprogs file gettext
+      gzip libcaca poppler_utils rpm sng sqlite squashfsTools unzip vim xz
+    ] ++ lib.optionals enableBloat [ jdk ghc fpc gnupg1 pdftk mono ];
 
   doCheck = false; # Calls 'mknod' in squashfs tests, which needs root