build: lint commit message in separate Travis job

Move the first commit message linting to a separate Travis job. Run the
script in bash debug mode to capture any issues communicating with the
GitHub API (e.g. network issues, rate limits).

PR-URL: #24254
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>

.travis.yml

@@ -4,17 +4,21 @@ cache: ccache
 os: linux
 matrix:
   include:
+    - name: "First commit message adheres to guidelines at <a href=\"https://goo.gl/p2fr5Q\">https://goo.gl/p2fr5Q</a>"
+      if: type = pull_request
+      language: node_js
+      node_js: "node"
+      script:
+        - if [ "${TRAVIS_PULL_REQUEST}" != "false" ]; then
+            bash -x tools/lint-pr-commit-message.sh ${TRAVIS_PULL_REQUEST};
+          fi
     - name: "Linter"
       language: node_js
       node_js: "node"
       env:
         - NODE=$(which node)
       script:
         - make lint
-        # Lint the first commit in the PR.
-        - if [ "${TRAVIS_PULL_REQUEST}" != "false" ]; then
-            bash tools/lint-pr-commit-message.sh ${TRAVIS_PULL_REQUEST} || true;
-          fi
     - name: "Test Suite"
       addons:
         apt: