support parsing of all hashes for pdiff

support parsing of all hashes for pdiff

The fileformat of a pdiff index stores currently only SHA1 hashes. With
this change, we look for all other hashes we support as well and take
what we get, so that we can work after the release of jessie to get
right of SHA1 if we want to.

Note that the completely patched file is and was checked against the
hashes collected from the Release file, so this transition isn't mission
critical.