message: Minor fix in TRACE
  Summary: Just added some comment in handling TRACEFILE and cleaned up the stderr case.
diff: @@ -52,7 +52,7 @@ struct Tracer {
  }
  ~Tracer() {
  - if (m_file) {
  + if (m_file != nullptr && m_file != stderr) {
  fclose(m_file);
  }
  }
  @@ -118,12 +118,17 @@ struct Tracer {
  void init_trace_file(const char* envfile) {
  if (!envfile) {
  - envfile = "/dev/stderr";
  + m_file = stderr;
  + return;
  }
  try {
  + // If redex-all is called from redex.py, the tracefile has been created
  + // already. And TRACEFILE is replaced by the file descriptor instead.
  + // Refer to update_trace_file in pyredex/logger.py.
  auto fd = std::stoi(envfile);
  m_file = fdopen(fd, "w");
  } catch (std::invalid_argument&) {
  + // Not an integer file descriptor; real file name.
  m_file = fopen(envfile, "w");
  }
  if (!m_file) {
  