workflows: add signed-off check

This adds a github workflow for checking the signed off line in commit
messages.
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

.github/workflows/signed-off.yml

+name: signed-off
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - run: "${GITHUB_WORKSPACE}/tests/scripts/workflows/signed-off.sh"
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tests/scripts/workflows/signed-off.sh

+#!/bin/bash
+set -x
+
+if [[ "$(git log --oneline --no-merges origin/"${GITHUB_BASE_REF}"..HEAD | wc -l)" -ne "$(git log --no-merges origin/"${GITHUB_BASE_REF}"..HEAD | grep -c Signed-off-by)" ]]; then
+  echo "One or more commits is/are missing a Signed-off-by. Add it with 'git commit -s'."
+  exit 1
+else
+  echo "Sign-off ok!"
+fi
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