Add rebase workflow (#1467)

.github/workflows/rebase.yml

@@ -0,0 +1,25 @@
+name: Automatic Rebase
+on:
+  issue_comment:
+    types: [created]
+jobs:
+  rebase:
+    name: Rebase
+    if: >-
+      github.event.issue.pull_request != '' && 
+      (
+        contains(github.event.comment.body, '/rebase') || 
+        contains(github.event.comment.body, '/autosquash')
+      )
+    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/rebase@1.7
+        with:
+          autosquash: ${{ contains(github.event.comment.body, '/autosquash') || contains(github.event.comment.body, '/rebase-autosquash') }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}