add automatic rebase

.github/workflows/rebase.yml

@@ -0,0 +1,34 @@
+ on:
+  check_run:
+    types:
+      # Check runs completing successfully can unblock the
+      # corresponding pull requests and make them mergeable.
+      - completed
+  pull_request:
+    types:
+      # A closed pull request makes the checks on the other
+      # pull request on the same base outdated.
+      - closed
+  pull_request_review:
+    types:
+      # Review approvals can unblock the pull request and
+      # make it mergeable.
+      - submitted
+  # Success statuses can unblock the corresponding
+  # pull requests and make them mergeable.
+  status: {}
+name: Automatic Rebase
+jobs:
+  rebase:
+    name: Rebase
+    if: github.event.issue.pull_request != ''
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout the latest code
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: Automatic Rebase
+      uses: cirrus-actions/rebase@1.3.1
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}