build: only run commit message checks on PRs

Splits out the commit message check so it only runs against pull requests. If an invalid
commit has made it into the main branch, we can't do much about it anymore so
the failure generates unnecessary noise.

Author: crisbeto

.circleci/config.yml

@@ -336,7 +336,6 @@ jobs:
             yarn check-entry-point-setup $(bazel info bazel-bin)/entry_points_manifest.json
 
       - run: yarn ng-dev format changed --check << pipeline.git.base_revision >>
-      - run: yarn ng-dev commit-message validate-range --range << pipeline.git.base_revision >>...<<pipeline.git.revision>>
       - run: yarn ownerslint
       - run: yarn stylelint
       - run: yarn tslint
@@ -345,6 +344,18 @@ jobs:
       - *slack_notify_on_failure
       - *save_cache
 
+  # -------------------------------------------------------------------------------------------
+  # Job to check that the commit message is formatted correctly.
+  # -------------------------------------------------------------------------------------------
+  check_commit_message:
+    <<: *job_defaults
+    steps:
+      - *checkout_code
+      - *restore_cache
+      - *yarn_install
+
+      - run: yarn ng-dev commit-message validate-range --range << pipeline.git.base_revision >>...<<pipeline.git.revision>>
+
   # -------------------------------------------------------------------------------------------
   # Job that builds all release packages. The built packages can be then used in the same
   # workflow to publish snapshot builds.
@@ -658,6 +669,10 @@ workflows:
           filters: *only_on_pull_requests_filter
           requires:
             - build_release_packages
+      - check_commit_message:
+          # Only run the commit message check on PRs, because if the
+          # commit made it into the main branch, it's already too late.
+          filters: *only_on_pull_requests_filter
       - lint:
           filters: *ignore_presubmit_branch_filter
       - ngcc_compatibility: