ci: progressively deepen fetch for merge-base reachability

Instead of a fixed --depth=200 fetch that can miss the merge base
for long-lived or stale branches, try 200 → 1000 → unshallow until
git merge-base succeeds. Most PRs resolve at depth 200 (no extra
cost); only old branches need the deeper fetches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet

.github/workflows/pr-build-main.yml

@@ -84,9 +84,24 @@ jobs:
         if: ${{ !inputs.skip_full_build }}
         run: |
           # Scalpel needs the merge base between HEAD and the base branch.
-          # The checkout is depth=1, so deepen both sides for merge-base reachability.
-          git fetch --deepen=200 2>/dev/null || true
-          git fetch --no-tags --depth=200 origin "${GITHUB_BASE_REF:-main}:refs/remotes/origin/${GITHUB_BASE_REF:-main}"
+          # The checkout is depth=1, so we progressively deepen until the
+          # merge base is reachable: 200 → 1000 → full history.
+          BASE_REF="${GITHUB_BASE_REF:-main}"
+          for depth in 200 1000; do
+            git fetch --deepen=$depth 2>/dev/null || true
+            git fetch --no-tags --depth=$depth origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
+            if git merge-base HEAD "origin/${BASE_REF}" >/dev/null 2>&1; then
+              echo "Merge base reachable at depth $depth"
+              break
+            fi
+            echo "Merge base not reachable at depth $depth, deepening..."
+          done
+          # If still not reachable, fetch full history as last resort
+          if ! git merge-base HEAD "origin/${BASE_REF}" >/dev/null 2>&1; then
+            echo "Merge base still not reachable, fetching full history"
+            git fetch --unshallow 2>/dev/null || true
+            git fetch --no-tags origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
+          fi
       - id: install-packages
         uses: ./.github/actions/install-packages
       - id: install-mvnd