get pdiff files from the same mirror as the index

get pdiff files from the same mirror as the index

In ad9416611ab83f7799f2dcb4bf7f3ef30e9fe6f8 we fall back to asking the
original mirror (e.g. a redirector) if we do not get the expected
result. This works for the indexes, but patches are a different beast
and much simpler. Adding this fallback code here seems like overkill as
they are usually right along their Index file, so actually forward the
relevant settings to the patch items which fixes pdiff support combined
with a redirector and partial mirrors as in such a situation the pdiff
patches would be 404 and the complete index would be downloaded.