message: Fix CopyFileTo when there is already a destination file.
  This fixes reindexing files, as before we would update the json but not
  the indexed file contents which caused code lens and references to get
  out of sync.
diff: @@ -189,7 +189,7 @@ void CopyFileTo(const std::string& dest, const std::string& source) {
  if (fd_from < 0)
  return;
  - int fd_to = open(dest.c_str(), O_WRONLY | O_CREAT | O_EXCL, 0666);
  + int fd_to = open(dest.c_str(), O_WRONLY | O_CREAT, 0666);
  if (fd_to < 0)
  goto out_error;
  