ci: add manual sync-from-upstream workflow

.gitea/workflows/sync-upstream.yml

@@ -0,0 +1,34 @@
+name: Sync from upstream
+
+on:
+  workflow_dispatch: {}
+
+jobs:
+  sync:
+    runs-on: [self-hosted-ubuntu]
+    timeout-minutes: 120
+    steps:
+      - name: Mirror-fetch from upstream and push here
+        run: |
+          # Manual-only trigger is load-bearing: this repo exists
+          # specifically so chromium-fork's CI never talks to
+          # https://chromium.googlesource.com/external/github.com/llvm/llvm-project directly (see chromium-fork's ci.yml
+          # url.insteadOf rewrite). Upstream only gets contacted when a
+          # human explicitly runs this.
+          set -e
+          WORK_DIR="$(mktemp -d)"
+          cd "$WORK_DIR"
+
+          git clone --mirror \
+            https://chromium.googlesource.com/external/github.com/llvm/llvm-project \
+            upstream.git
+
+          SERVER_HOST="${{ github.server_url }}"
+          SERVER_HOST="${SERVER_HOST#https://}"
+          REPO_URL="https://x-access-token:${{ secrets.GITHUB_TOKEN }}@${SERVER_HOST}/${{ github.repository }}.git"
+
+          cd upstream.git
+          git push --mirror "$REPO_URL"
+
+          cd "$WORK_DIR"
+          rm -rf upstream.git