Fixups to commit message checks to only happen during PR pushes (#509)

Handles bugs from #508.

Also try and fix a quoting issues.

.github/workflows/devcontainer.yml

@@ -29,12 +29,17 @@ jobs:
 
     - name: Get commit messages
       id: get-commit-messages
+      if: github.event_name == 'pull_request'
       run: |
         # Only look at the last 10 commits, to avoid getting too much data.
         # If the message wasn't in there (e.g. because the push was a merge and too large), then we just ignore it.
         git fetch --deepen=10
-        git log --format="%B" -10 ${{ github.event.before }}..${{ github.event.after }} > /tmp/commit_messages.txt || true
-        echo "COMMIT_MESSAGES=$(cat /tmp/commit_messages.txt)" >> $GITHUB_OUTPUT
+        # Handle multiline output:
+        {
+          echo 'COMMIT_MESSAGES<<COMMIT_MESSAGES_EOF'
+          git log --format="%B" -10 ${{ github.event.before }}..${{ github.event.after }} | tee /tmp/commit_messages.txt
+          echo COMMIT_MESSAGES_EOF
+        } >> $GITHUB_OUTPUT
 
     - name: Set NO_CACHE variable based on commit messages and for nightly builds
       if: github.event_name == 'schedule' || contains(steps.get-commit-messages.outputs.COMMIT_MESSAGES, 'NO_CACHE=true')
@@ -46,6 +51,9 @@ jobs:
         set -x
         printenv
         echo "NO_CACHE: ${NO_CACHE:-}"
+        echo "EVENT_NAME: ${{ github.event_name }}"
+        echo "BEFORE: ${{ github.event.before }}"
+        echo "AFTER: ${{ github.event.after }}"
         echo "COMMIT_MESSAGES: ${{ steps.get-commit-messages.outputs.COMMIT_MESSAGES }}"
         echo "COMMIT_SHA: ${{ github.sha }}"
         echo "git log -1 ${{ github.sha }}: $(git log -1 ${{ github.sha }})"