Avoid potentially unnecessary string reallocs.

In case `dest` is not created using a subset of the `input`
`dest.reserve(input.size())` would result in increasing `dest`s
size with likely reallocation. On top of being less efficient
new version also generates significantly less asm with GCC 13.
https://compiler-explorer.com/z/x3avEdWGM

src/unicode.cpp

@@ -383,8 +383,9 @@ std::string percent_decode(const std::string_view input, size_t first_percent) {
   if (first_percent == std::string_view::npos) {
     return std::string(input);
   }
-  std::string dest(input.substr(0, first_percent));
+  std::string dest;
   dest.reserve(input.length());
+  dest.append(input.substr(0, first_percent));
   const char* pointer = input.data() + first_percent;
   const char* end = input.data() + input.size();
   // Optimization opportunity: if the following code gets