* New primop builtins.filterSource, which can be used to filter files

from a source directory.  All files for which a predicate function
  returns true are copied to the store.  Typical example is to leave
  out the .svn directory:

    stdenv.mkDerivation {
      ...
      src = builtins.filterSource
        (path: baseNameOf (toString path) != ".svn")
        ./source-dir;
      # as opposed to
      #   src = ./source-dir;
    }

  This is important because the .svn directory influences the hash in
  a rather unpredictable and variable way.

src/libutil/hash.cc

@@ -294,13 +294,13 @@ struct HashSink : Sink
 };
 
 
-Hash hashPath(HashType ht, const Path & path)
+Hash hashPath(HashType ht, const Path & path, PathFilter & filter)
 {
     HashSink sink;
     sink.ht = ht;
     Hash hash(ht);
     start(ht, sink.ctx);
-    dumpPath(path, sink);
+    dumpPath(path, sink, filter);
     finish(ht, sink.ctx, hash.hash);
     return hash;
 }