Fix workflow filter on pull-request event (#4550)

Author: juergba

.github/workflows/mocha.yml

@@ -6,14 +6,13 @@ name: Tests
       - opened
       - synchronize
       - reopened
-    branches:
-      # Branches from forks have the form 'user:branch-name'
-      - '**:**'
 
 jobs:
   prepare-commit-msg:
     name: Retrieve head commit message
     runs-on: ubuntu-latest
+    # Run 'pull-request' event only on external PRs from forked repos.
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
     outputs:
       HEAD_COMMIT_MSG: '${{ steps.commitMsg.outputs.HEAD_COMMIT_MSG }}'
     steps: