GitLab CI: Add plumbing for merge request feedback

Install the GitLb CLI in the Commit Check job and add warnings.

.gitlab-ci.yml

@@ -649,12 +649,44 @@ Commit Check:
   extends: .build-ubuntu
   rules: !reference [.if-merge-request]
   script:
+    - (cd /tmp ; curl -JLO https://gitlab.com/gitlab-org/cli/-/releases/v1.34.0/downloads/glab_1.34.0_Linux_x86_64.deb)
+    - dpkg --install /tmp/glab_1.34.0_Linux_x86_64.deb
+    - glab auth status
+    - if [[ $NUM_COMMITS > 1 ]] ; then glab mr note $CI_MERGE_REQUEST_IID --unique --message "ℹ️ This merge request has multiple commits." ; fi
     # build-ubuntu puts us in `build`.
     - cd ..
     - git status
-    - bash ./tools/pre-commit "${CI_COMMIT_SHA}~$NUM_COMMITS"
-    - tools/validate-commit.py
-    - python3 tools/checklicenses.py
+    - PC_EXIT_CODE=0
+    - ANALYSIS_MESSAGE=$( ./tools/pre-commit "${CI_COMMIT_SHA}~$NUM_COMMITS" || PC_EXIT_CODE=1 )
+    - >
+      if [ -n "$ANALYSIS_MESSAGE" ] ; then
+        glab mr note $CI_MERGE_REQUEST_IID --unique --message "Pre-commit check:
+        ~~~
+        $ANALYSIS_MESSAGE
+        ~~~
+        "
+      fi
+    - VC_EXIT_CODE=0
+    - ANALYSIS_MESSAGE=$( tools/validate-commit.py || VC_EXIT_CODE=1 )
+    - >
+      if [[ $VC_EXIT_CODE != 0 ]] ; then
+        glab mr note $CI_MERGE_REQUEST_IID --unique --message "Commit validation:
+        ~~~
+        $ANALYSIS_MESSAGE
+        ~~~
+        "
+      fi
+    - LC_EXIT_CODE=0
+    - ANALYSIS_MESSAGE=$( python3 tools/checklicenses.py || LC_EXIT_CODE=1 )
+    - >
+      if [[ $LC_EXIT_CODE != 0 ]] ; then
+        glab mr note $CI_MERGE_REQUEST_IID --unique --message "License check:
+        ~~~
+        $ANALYSIS_MESSAGE
+        ~~~
+        "
+      fi
+    - exit $(( PC_EXIT_CODE || VC_EXIT_CODE || LC_EXIT_CODE ))
 
 Ubuntu GCC Build:
   extends: .build-ubuntu