QList::append(QList) doesn't require a temp copy

The method can simply forward to the append overload taking
iterators. That method is safe against the iterators being
part of the list itself.

Change-Id: I4bebd6c1118cd4a428fa9248235029b997ef60b2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/corelib/tools/qlist.h

@@ -322,9 +322,7 @@ public:
     }
     void append(const QList<T> &l)
     {
-        // protect against l == *this
-        QList list(l);
-        append(list.constBegin(), list.constEnd());
+        append(l.constBegin(), l.constEnd());
     }
     void append(QList<T> &&l);
     void prepend(rvalue_ref t) {