github: workflows: compliance: Ensure no merge commits

Add a simple one-liner that checks for "0" in the count of merge
commits. If a merge commit is present the output will be "1"
or higher, failing the job.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

.github/workflows/compliance.yml

@@ -37,6 +37,9 @@ jobs:
         git config --global user.email "you@example.com"
         git config --global user.name "Your Name"
         git remote -v
+        # Ensure there's no merge commits in the PR
+        [[ "$(git rev-list --merges --count origin/${BASE_REF}..)" == "0" ]] || \
+        (echo "::error ::Merge commits not allowed, rebase instead";false)
         git rebase origin/${BASE_REF}
         # debug
         git log  --pretty=oneline | head -n 10