logsink: fix multiple issues with LogSink::ToString()

1. Initializing std::ostringstream with a string makes no sense, as the
   string becomes an initial value of an underlying buffer; seek-to-end
   is not performed, so the initial value gets completely overwritten by
   subsequent writing.

2. Flag `log_year_in_prefix` should be considered, as if formatting a
   regular logging message.

3. Writing a buffer to std::ostream is better expressed with write(s,n).

src/logging.cc

@@ -2115,12 +2115,14 @@ void LogSink::WaitTillSent() {
 string LogSink::ToString(LogSeverity severity, const char* file, int line,
                          const LogMessageTime& logmsgtime, const char* message,
                          size_t message_len) {
-  ostringstream stream(string(message, message_len));
+  ostringstream stream;
   stream.fill('0');
 
-  stream << LogSeverityNames[severity][0]
-         << setw(4) << 1900 + logmsgtime.year()
-         << setw(2) << 1 + logmsgtime.month()
+  stream << LogSeverityNames[severity][0];
+  if (FLAGS_log_year_in_prefix) {
+    stream << setw(4) << 1900 + logmsgtime.year();
+  }
+  stream << setw(2) << 1 + logmsgtime.month()
          << setw(2) << logmsgtime.day()
          << ' '
          << setw(2) << logmsgtime.hour() << ':'
@@ -2132,7 +2134,14 @@ string LogSink::ToString(LogSeverity severity, const char* file, int line,
          << ' '
          << file << ':' << line << "] ";
 
-  stream << string(message, message_len);
+#ifdef _MSC_VER
+#pragma push_macro("_write")
+#undef _write
+#endif  // _MSC_VER
+  stream.write(message, static_cast<std::streamsize>(message_len));
+#ifdef _MSC_VER
+#pragma pop_macro("_write")
+#endif  // _MSC_VER
   return stream.str();
 }