Revision 1914656

Move some utility class instantiations into the DiffGenerator.

* tools/hook-scripts/mailer/mailer.py:
  (generate_content): do not bother with creating DiffSelections or
    DiffURLSelections instances, nor pass such. The DiffGenerator will
    take care of it now, internally.
  (DiffGenerator.__init__): remove the diffsels and diffurls args. We
    can easily create this objects ourself. Do so.
    Non-sequitur: remove .diff and .diff_url instance variables. These
      are unused; a conflation with diff chunks from our generator.