message: Fix clang 3.5 compile error
diff: @@ -286,10 +286,10 @@ std::vector<FileContents> PreloadFileContents(
  // index->file_contents comes from cache, so we need to check if that cache is
  // still valid. if so, we can use it, otherwise we need to load from disk.
  auto get_latest_content = [](const std::string& path, int64_t cached_time,
  - const std::string& cached) {
  + const std::string& cached) -> std::string {
  optional<int64_t> mod_time = GetLastModificationTime(path);
  if (!mod_time)
  - return std::string("");
  + return "";
  if (*mod_time == cached_time)
  return cached;
  @@ -297,7 +297,7 @@ std::vector<FileContents> PreloadFileContents(
  optional<std::string> fresh_content = ReadContent(path);
  if (!fresh_content) {
  LOG_S(ERROR) << "Failed to load content for " << path;
  - return std::string("");
  + return "";
  }
  return *fresh_content;
  };
  