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.

apt-pkg/acquire-item.cc

@@ -1837,10 +1837,18 @@ bool pkgAcqDiffIndex::ParseDiffIndex(string const &IndexDiffFile)	/*{{{*/
       std::clog << "Server-Current: " << ServerHashes.find(NULL)->toStr() << " and we start at "
 	 << CurrentPackagesFile << " " << LocalHashes.FileSize() << " " << LocalHashes.find(NULL)->toStr() << std::endl;
 
+   // historically, older hashes have more info than newer ones, so start
+   // collecting with older ones first to avoid implementing complicated
+   // information merging techniques… a failure is after all always
+   // recoverable with a complete file and hashes aren't changed that often.
+   std::vector<char const *> types;
+   for (char const * const * type = HashString::SupportedHashes(); *type != NULL; ++type)
+      types.push_back(*type);
+
    // parse all of (provided) history
    vector<DiffInfo> available_patches;
    bool firstAcceptedHashes = true;
-   for (char const * const * type = HashString::SupportedHashes(); *type != NULL; ++type)
+   for (auto type = types.crbegin(); type != types.crend(); ++type)
    {
       if (LocalHashes.find(*type) == NULL)
 	 continue;
@@ -1898,7 +1906,7 @@ bool pkgAcqDiffIndex::ParseDiffIndex(string const &IndexDiffFile)	/*{{{*/
       return false;
    }
 
-   for (char const * const * type = HashString::SupportedHashes(); *type != NULL; ++type)
+   for (auto type = types.crbegin(); type != types.crend(); ++type)
    {
       if (LocalHashes.find(*type) == NULL)
 	 continue;
@@ -1938,7 +1946,7 @@ bool pkgAcqDiffIndex::ParseDiffIndex(string const &IndexDiffFile)	/*{{{*/
       }
    }
 
-   for (char const * const * type = HashString::SupportedHashes(); *type != NULL; ++type)
+   for (auto type = types.crbegin(); type != types.crend(); ++type)
    {
       std::string tagname = *type;
       tagname.append("-Download");