Set up a GitHub Action to keep in sync with Cygwin

This will help us by automating an otherwise tedious task.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/sync-with-cygwin.yml

@@ -0,0 +1,24 @@
+name: sync-with-cygwin
+
+# File: .github/workflows/repo-sync.yml
+
+on:
+  workflow_dispatch:
+  schedule:
+  - cron:  "42 * * * *"
+jobs:
+  repo-sync:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+    - name: Fetch Cygwin's latest master and tags
+      run: |
+        git init --bare
+        # Potentially use git://sourceware.org/git/newlib-cygwin.git directly, but GitHub seems more reliable
+        git fetch https://github.com/cygwin/cygwin master:refs/heads/cygwin/master 'refs/tags/*:refs/tags/*'
+    - name: Push to our fork
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        git push https://$GITHUB_ACTOR:$GITHUB_TOKEN@github.com/$GITHUB_REPOSITORY refs/heads/cygwin/master 'refs/tags/*:refs/tags/*'