refactor: Preallocate PrevectorFillVector(In)Direct without vector resize

The prevector benchmarks were likely not trying to measure vector resize performance.

Author: l0rinc

src/bench/prevector.cpp

@@ -87,6 +87,7 @@ static void PrevectorFillVectorDirect(benchmark::Bench& bench)
 {
     bench.run([&] {
         std::vector<prevector<28, T>> vec;
+        vec.reserve(260);
         for (size_t i = 0; i < 260; ++i) {
             vec.emplace_back();
         }
@@ -99,6 +100,7 @@ static void PrevectorFillVectorIndirect(benchmark::Bench& bench)
 {
     bench.run([&] {
         std::vector<prevector<28, T>> vec;
+        vec.reserve(260);
         for (size_t i = 0; i < 260; ++i) {
             // force allocation
             vec.emplace_back(29, T{});