Enable Automatic Rebase: Fix #8927 (#9018)

* Enable Automatic Rebase: Fix #8927

* Update rebase.yml to use startsWith

Use startsWith() instead of contains() in Github Action so as to prevent accidental Rebase of PRs.

.github/workflows/rebase.yml

@@ -0,0 +1,18 @@
+# https://github.com/marketplace/actions/automatic-rebase  (https://github.com/cirrus-actions/rebase)
+on:
+  issue_comment:
+    types: [created]
+name: Automatic Rebase
+jobs:
+  rebase:
+    name: Rebase
+    if: github.event.issue.pull_request != '' && startsWith(github.event.comment.body, '/rebase') && github.event.comment.author_association == 'MEMBER'
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+      with:
+        fetch-depth: 0
+    - name: Automatic Rebase
+      uses: cirrus-actions/rebase@1.3.1
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}