message: Add ability to run clang-tidy against specific ref
diff: @@ -55,6 +55,9 @@ function filter_excludes() {
  exclude_testdata | exclude_chromium_url | exclude_win32_impl
  }
  +if [[ -z "${DIFF_REF}" && "${BUILD_REASON}" != "PullRequest" ]]; then
  + DIFF_REF=HEAD^
  +fi
  if [[ "${RUN_FULL_CLANG_TIDY}" == 1 ]]; then
  echo "Running full clang-tidy..."
  @@ -64,9 +67,9 @@ if [[ "${RUN_FULL_CLANG_TIDY}" == 1 ]]; then
  -export-fixes=${FIX_YAML} \
  -j ${NUM_CPUS:-0} -p 1 -quiet \
  ${APPLY_CLANG_TIDY_FIXES:+-fix}
  -elif [[ "${BUILD_REASON}" != "PullRequest" ]]; then
  - echo "Running clang-tidy-diff against previous commit..."
  - git diff HEAD^ | filter_excludes | \
  +elif [[ -n "${DIFF_REF}" ]]; then
  + echo "Running clang-tidy-diff against ref ${DIFF_REF}"
  + git diff ${DIFF_REF} | filter_excludes | \
  python3 "${LLVM_PREFIX}/share/clang/clang-tidy-diff.py" \
  -clang-tidy-binary=${CLANG_TIDY} \
  -export-fixes=${FIX_YAML} \
  