message: Refactor formatting lambda.
diff: @@ -45,11 +45,9 @@ EmissionContext::EmissionContext(std::unique_ptr<HloModule> module)
  [&instructions_with_error](const HloInstruction* instr,
  const string& instr_name, int indent,
  bool is_root) {
  - string tab;
  - for (int i = 0; i < indent; i++) {
  - tab += " ";
  - }
  - string result = tab + (is_root ? "ROOT " : "") + instr_name;
  + const string tab(2 * indent, ' ');
  + string result =
  + absl::StrCat(tab, is_root ? "ROOT " : "", instr_name);
  if (!instructions_with_error.count(instr)) {
  return result;
  }
  