waste less time in mkdirp

src/sys.cc

 #include <sys/wait.h>
 #include <stdexcept>
 #include <system_error>
+#include <unordered_set>
 
 #include "logger.h"
 #include "globals.h"
 }
 
 void mkdirp(const std::string &path) {
+	static thread_local std::unordered_set<std::string> set;
+	auto res = set.emplace(path);
+	if (!res.second)
+		return;
+
 	// TODO: Implement this in C++ instead
 	std::vector<std::string> argv;
 	argv.push_back("mkdir");