Use a new container in each round of push_back test.

test/benchmark_test.cc

@@ -86,11 +86,12 @@ BENCHMARK(BM_SetInsert)->RangePair(1<<10,8<<10, 1,10);
 
 template<typename Q>
 static void BM_Sequential(benchmark::State& state) {
+  typename Q::value_type v = 42;
+  while (state.KeepRunning()) {
     Q q;
-  typename Q::value_type v;
-  while (state.KeepRunning())
     for (int i = state.range_x(); --i; )
       q.push_back(v);
+  }
   const int64_t items_processed =
       static_cast<int64_t>(state.iterations()) * state.range_x();
   state.SetItemsProcessed(items_processed);