hide first pdiff merge failure debug message

The sibling of this message are all guarded as debug messages, just this
one had it missing an subsequently causes display issues if triggered.

Git-Dch: Ignore

apt-pkg/acquire-item.cc

@@ -1044,7 +1044,8 @@ void pkgAcqIndexMergeDiffs::Failed(string Message,pkgAcquire::MethodConfig * Cnf
 
    // first failure means we should fallback
    State = StateErrorDiff;
-   std::clog << "Falling back to normal index file acquire" << std::endl;
+   if (Debug)
+      std::clog << "Falling back to normal index file acquire" << std::endl;
    new pkgAcqIndex(Owner, TransactionManager, Target, ExpectedHashes, MetaIndexParser);
 }
 									/*}}}*/