Add Automatic Rebase Action

See https://github.com/marketplace/actions/automatic-rebase

.github/workflows/rebase.yaml

@@ -0,0 +1,21 @@
+name: Automatic Rebase
+on:
+  issue_comment:
+    types: [created]
+jobs:
+  rebase:
+    name: Rebase
+    if: |
+      (github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase'))
+      && (github.event.comment.author_association == 'MEMBER')
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout the latest code
+      uses: actions/checkout@v2
+      with:
+        token: ${{ secrets.GITHUB_TOKEN }}
+        fetch-depth: 0 # otherwise, you will fail to push refs to dest repo
+    - name: Automatic Rebase
+      uses: cirrus-actions/[email protected]
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}