CI: add base-commit fetch helper

.github/actions/ensure-base-commit/action.yml

@@ -0,0 +1,47 @@
+name: Ensure base commit
+description: Ensure a shallow checkout has enough history to diff against a base SHA.
+inputs:
+  base-sha:
+    description: Base commit SHA to diff against.
+    required: true
+  fetch-ref:
+    description: Branch or ref to deepen/fetch from origin when base-sha is missing.
+    required: true
+runs:
+  using: composite
+  steps:
+    - name: Ensure base commit is available
+      shell: bash
+      env:
+        BASE_SHA: ${{ inputs.base-sha }}
+        FETCH_REF: ${{ inputs.fetch-ref }}
+      run: |
+        set -euo pipefail
+
+        if [ -z "$BASE_SHA" ] || [[ "$BASE_SHA" =~ ^0+$ ]]; then
+          echo "No concrete base SHA available; skipping targeted fetch."
+          exit 0
+        fi
+
+        if git rev-parse --verify "$BASE_SHA^{commit}" >/dev/null 2>&1; then
+          echo "Base commit already present: $BASE_SHA"
+          exit 0
+        fi
+
+        for deepen_by in 25 100 300; do
+          echo "Base commit missing; deepening $FETCH_REF by $deepen_by."
+          git fetch --no-tags --deepen="$deepen_by" origin "$FETCH_REF" || true
+          if git rev-parse --verify "$BASE_SHA^{commit}" >/dev/null 2>&1; then
+            echo "Resolved base commit after deepening: $BASE_SHA"
+            exit 0
+          fi
+        done
+
+        echo "Base commit still missing; fetching full history for $FETCH_REF."
+        git fetch --no-tags origin "$FETCH_REF" || true
+        if git rev-parse --verify "$BASE_SHA^{commit}" >/dev/null 2>&1; then
+          echo "Resolved base commit after full ref fetch: $BASE_SHA"
+          exit 0
+        fi
+
+        echo "Base commit still unavailable after fetch attempts: $BASE_SHA"