Add new workflow to fixup backports for release notes (#3252)

The Mergify-generated backports do not currently include the information
needed by the release notes generation automation. This new workflow
will copy such information over to backport PRs.

(cherry picked from commit 86ccd1b)

.github/workflows/backport-fixup.yml

@@ -0,0 +1,90 @@
+# Fixes up Mergify-created backport PRs to include necessary labels and other
+# information for release notes generation
+
+name: Backport Fixup
+
+on:
+  pull_request:
+    types: [opened]
+  workflow_dispatch:
+    inputs:
+      pr:
+        description: 'Number of the Pull Request to Fixup'
+        require: true
+
+permissions:
+  pull-requests: write
+  contents: write
+
+jobs:
+  resolve_prs:
+    name: Resolve PRs
+    runs-on: ubuntu-latest
+    # If triggering PR actually is a backport, then original_pr will be set
+    outputs:
+        backport_pr: ${{ steps.backport.outputs.pr }}
+        original_pr: ${{ steps.original.outputs.pr }}
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Figure out backport PR number
+        id: backport
+        run: |
+          if [[ -z "${{ inputs.pr }}" ]]; then
+            echo "pr=${{ github.event.number }}" >> "$GITHUB_OUTPUT"
+          else
+            echo "pr=${{ inputs.pr }}" >> "$GITHUB_OUTPUT"
+          fi
+      - name: Figure out original PR number (if one exists)
+        id: original
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          BP_PR=${{ steps.backport.outputs.pr }}
+          TITLE=$(gh pr view --json title --jq '.title' $BP_PR)
+          if [[ "$TITLE" =~ \(backport\ #([0-9]+)\) ]]; then
+            echo "pr=${BASH_REMATCH[1]}" >> "$GITHUB_OUTPUT"
+          else
+            echo "$BP_PR is not a backport PR!" >> $GITHUB_STEP_SUMMARY
+          fi
+
+  fixup_backport:
+    name: Fixup the backport PR
+    runs-on: ubuntu-latest
+    needs: [resolve_prs]
+    if: ${{ needs.resolve_prs.outputs.original_pr }}
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Copy over labels
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          BP_PR=${{ needs.resolve_prs.outputs.backport_pr }}
+          ORIG_PR=${{ needs.resolve_prs.outputs.original_pr }}
+          LABELS=$(gh pr view --json labels --jq '.labels | .[].name | select(. != "Backported")' $ORIG_PR)
+          for label in $LABELS; do
+            gh pr edit $BP_PR --add-label $label
+          done
+      - name: Copy over body
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          BP_PR=${{ needs.resolve_prs.outputs.backport_pr }}
+          ORIG_PR=${{ needs.resolve_prs.outputs.original_pr }}
+
+          gh pr view --json body --jq '.body' $ORIG_PR > orig_body.txt
+          gh pr view --json body --jq '.body' $BP_PR > bp_body.txt
+
+          if grep -q '# Original PR Body' bp_body.txt; then
+            # Copy BP PR body but remove original PR body from bottom
+            sed '/# Original PR Body/q' bp_body.txt > new_bp_body.txt
+            echo "" >> new_bp_body.txt
+          else
+            cp bp_body.txt new_bp_body.txt
+            echo -e "\n----\n\n# Original PR Body\n" >> new_bp_body.txt
+          fi
+
+          cat orig_body.txt >> new_bp_body.txt
+
+          gh pr edit $BP_PR --body-file new_bp_body.txt