[theme-reviewers] Update diffs

[Otto]

I've added code to the theme uploader that will attempt to add a link
into the trac description that goes to the changeset diff on trac
(which Phil pointed me to) for update theme uploads.

But I don't have a good way to test this, so the link may be wrong in
some way I didn't think of. Tell me if you spot any flaws with it in
updated theme uploads (starting from today).

For reference, the diff link should look like this (for example):
http://themes.trac.wordpress.org/changeset?old_path=/graphene/1.0.7&new_path=/graphene/1.0.8

It's only looking for the latest theme version, but it should be a
handy shortcut for reviewers.

-Otto