github: Add rebase label on PR if it's > 50 commits behind

The idea is to catch some PRs and rerun them in CI to avoid merging PRs that
are based on the stale base branch and causes issues afterwards.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

.github/workflows/behind-base.yml

@@ -0,0 +1,28 @@
+name: Add rebase label if the branch is > 50 commits behind
+
+on:
+  pull_request_target:
+    types: [synchronize, opened, reopened, labeled, unlabeled]
+
+jobs:
+  behind:
+    if: github.repository == 'frrouting/frr'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+      - name: Set custom variables
+        id: vars
+        run: |
+          echo "behind_by=$(git log --oneline origin/${{ github.base_ref }} ^${{ github.event.pull_request.head.sha }} | wc -l)" >> $GITHUB_OUTPUT
+      - name: Add rebase label if needed
+        if: ${{ steps.vars.outputs.behind_by > 50 }}
+        uses: actions-ecosystem/action-add-labels@v1
+        with:
+          labels: rebase