ci: derive changed files from commit

.forgejo/workflows/ci.yml

@@ -14,19 +14,50 @@ jobs:
     runs-on: docker
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
 
       - id: filter
-        uses: dorny/paths-filter@v3
-        with:
-          filters: |
-            backend:
-              - 'back001/**'
-              - '.forgejo/workflows/**'
-              - 'ci/**'
-            frontend:
-              - 'front001/**'
-              - '.forgejo/workflows/**'
-              - 'ci/**'
+        name: Detect changes
+        run: |
+          set -euo pipefail
+          git fetch --quiet --depth=1 origin "${GITHUB_BASE_REF:-}" || true
+
+          files=""
+          if [ "${GITHUB_EVENT_NAME:-}" = "pull_request" ] && [ -n "${GITHUB_BASE_REF:-}" ]; then
+            base_ref="origin/${GITHUB_BASE_REF}"
+            if git rev-parse --verify "$base_ref" >/dev/null 2>&1; then
+              files="$(git diff --name-only "$base_ref"...HEAD)" || true
+            else
+              files=""
+            fi
+          fi
+
+          if [ -z "$files" ]; then
+            if [ -n "${GITHUB_SHA:-}" ]; then
+              files="$(git show --name-only --pretty= "${GITHUB_SHA}")"
+            else
+              files="$(git show --name-only --pretty= HEAD)"
+            fi
+          fi
+
+          backend=false
+          frontend=false
+          while IFS= read -r path; do
+            case "$path" in
+              back001/*|.forgejo/workflows/*|ci/*) backend=true ;;
+            esac
+            case "$path" in
+              front001/*|.forgejo/workflows/*|ci/*) frontend=true ;;
+            esac
+          done <<EOF
+          $files
+          EOF
+
+          {
+            echo "backend=$backend"
+            echo "frontend=$frontend"
+          } >> "$GITHUB_OUTPUT"
     outputs:
       backend: ${{ steps.filter.outputs.backend }}
       frontend: ${{ steps.filter.outputs.frontend }}