Add rebase CI

Author: hasufell

.github/workflows/rebase.yaml

@@ -0,0 +1,42 @@
+name: Rebase against upstream
+
+on:
+  pull_request:
+    branches:
+      - master
+  schedule:
+    - cron: '15 3 * * *'
+
+jobs:
+  rebase:
+    name: Rebase now!
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - run: |
+          set -eux
+
+          base_branch=master
+          upstream_repo=https://github.com/haskell/cabal.git
+          upstream_branch=master
+
+          git remote add upstream "${upstream_repo}"
+          git fetch upstream
+          common_ancestor=$(git merge-base "${base_branch}" "upstream/${upstream_branch}")
+          git format-patch "${common_ancestor}".."${base_branch}"
+          git archive "${base_branch}" > backup.tar
+          tar -rf backup.tar .git
+          git am *.patch
+          git push -f origin master-rebased
+
+      - if: always()
+        name: Upload backup
+        uses: actions/upload-artifact@v4
+        with:
+          if-no-files-found: error
+          retention-days: 7
+          name: backup
+          path: |
+            ./backup.tar