ensure sha512 is really used when available (thanks to Tyler Hicks )

apt-pkg/acquire-item.cc

@@ -1730,7 +1730,7 @@ bool pkgAcqArchive::QueueNext()
       {
 	 if(stringcasecmp(ForceHash, "sha512") == 0)
 	    ExpectedHash = HashString("SHA512", Parse.SHA512Hash());
-	 if(stringcasecmp(ForceHash, "sha256") == 0)
+	 else if(stringcasecmp(ForceHash, "sha256") == 0)
 	    ExpectedHash = HashString("SHA256", Parse.SHA256Hash());
 	 else if (stringcasecmp(ForceHash, "sha1") == 0)
 	    ExpectedHash = HashString("SHA1", Parse.SHA1Hash());

cmdline/apt-get.cc

@@ -2395,7 +2395,7 @@ bool DoDownload(CommandLine &CmdL)
       HashString hash;
       if (rec.SHA512Hash() != "")
          hash = HashString("sha512", rec.SHA512Hash());
-      if (rec.SHA256Hash() != "")
+      else if (rec.SHA256Hash() != "")
          hash = HashString("sha256", rec.SHA256Hash());
       else if (rec.SHA1Hash() != "")
          hash = HashString("sha1", rec.SHA1Hash());