[repo] keep master in sync with main

Add an action to keep the old master branch up to date with main.

.github/workflows/master-mirrors-main.yaml

@@ -0,0 +1,19 @@
+name: 'sync master with main'
+
+on:
+  push:
+    branches:
+      - 'main'
+
+jobs:
+  push-master:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: Push HEAD to master
+      # Deliberately not force-pushing: designed to fail if this would mean rewriting history
+      run: |
+        git push origin HEAD:master