tools: apply linting to first commit in PRs

Use Travis-CI to check the formatting of the first commit in a pull
request. This will hopefully reduce formatting errors and nits about
them in pull requests.

.travis.yml

@@ -12,6 +12,8 @@ matrix:
         - NODE=$(which node)
       script:
         - make lint
+        # Lint the first commit in the PR.
+        - git log HEAD ^$TRAVIS_BRANCH --pretty=format:'%h' --no-merges | tail -1 | xargs npx core-validate-commit --no-validate-metadata
     - name: "Test Suite"
       install:
         - ./configure