parse .diff/Index hashes in reverse order

parse .diff/Index hashes in reverse order

Reversing the parsing order ensures that we parse weaker hashes (like
SHA1) before we touch newer/stronger hashes (like SHA256) as the weaker
ones will usually be there for a longer time already with data already
present, which we would discard if we start with the strong one first.

The discarding is visible in the debug logs:
File X wasn't in the list for the first parsed hash! (history)
File X wasn't in the list for the first parsed hash! (patches)
which if file X is part of the patch-path means apt will not find a path and
fallback to acquire the whole file instead needlessly.
If file X isn't part of the patch-path that is no problem, so that
effects only the update-call which updates with patches coming from
before and after the addition of a new hash.