* 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.

tests/filter-source.nix.in

@@ -0,0 +1,7 @@
+derivation {
+  name = "filter";
+  system = "@system@";
+  builder = "@shell@";
+  args = ["-e" "-x" (builtins.toFile "builder" "PATH=@testPath@; ln -s $input $out")];
+  input = builtins.filterSource (path: baseNameOf (toString path) != "foo") ./test-tmp/filterin;
+}