Add rebase CI

Author: hasufell

.github/rebase-error-issue.md

@@ -0,0 +1,4 @@
+---
+title: Rebase failed on {{ date | date('dddd, MMMM Do') }}
+---
+{{ env.WORKFLOW_URL }}

.github/workflows/rebase.yaml

@@ -0,0 +1,87 @@
+name: Rebase against upstream
+
+on:
+  pull_request:
+    branches:
+      - master
+  schedule:
+    - cron: '15 3 * * *'
+
+permissions:
+  contents: write
+
+jobs:
+  rebase:
+    name: Rebase now!
+    runs-on: ubuntu-latest
+    env:
+      BASE_BRANCH: release-ci
+      UPSTREAM_REPO: https://github.com/haskell/cabal.git
+      UPSTREAM_BRANCH: master
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ env.BASE_BRANCH }}
+          fetch-depth: 0
+          persist-credentials: false
+
+      - run: |
+          set -eux
+
+          base_branch=${{ env.BASE_BRANCH }}
+          upstream_repo=${{ env.UPSTREAM_REPO }}
+          upstream_branch=${{ env.UPSTREAM_BRANCH }}
+
+          # print version for debugging
+          git --version
+
+          # required to apply patches
+          git config user.email "[email protected]"
+          git config user.name "GitHub CI"
+
+          # create patchset from the diverging commits
+          git remote add upstream "${upstream_repo}"
+          git fetch upstream
+          git fetch origin
+          git status
+          common_ancestor=$(git merge-base "${base_branch}" "upstream/${upstream_branch}")
+          git format-patch "${common_ancestor}".."${base_branch}"
+
+          # backup full repo
+          git archive "${base_branch}" > backup.tar
+          tar -rf backup.tar .git
+
+          exit 1
+
+          # do the actual "rebase" and push
+          if compgen -G "*.patch" > /dev/null; then
+            git reset --hard "upstream/${upstream_branch}"
+            git am *.patch
+            git checkout -b master-rebased
+            git push -f https://${{ secrets.REBASE_PAT }}@github.com/stable-haskell/cabal.git master-rebased
+          fi
+        shell: bash
+
+      # create an issue with a link to the workflow run on failure
+      - if: failure()
+        run: git reset --hard ${{ github.ref }}
+      - if: failure()
+        uses: JasonEtco/create-an-issue@v2
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+          WORKFLOW_URL: "${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"
+        with:
+          filename: .github/rebase-error-issue.md
+
+      # we always want the original repo and the patches that were applied
+      # for debugging and backup reasons
+      - if: always()
+        name: Upload backup
+        uses: actions/upload-artifact@v4
+        with:
+          if-no-files-found: error
+          retention-days: 7
+          name: backup
+          path: |
+            ./backup.tar