Add automatic rebase command (#668)

* Add automatic rebase command

https://github.com/cirrus-actions/rebase

The command is limited to org member or repo collaborator.

* Use GH App token to trigger CI actions

.github/workflows/automatic-rebase.yaml

@@ -0,0 +1,35 @@
+name: Automatic Rebase
+on:
+  issue_comment:
+    types: [created]
+jobs:
+  rebase:
+    name: Rebase
+    runs-on: ubuntu-latest
+    if: >-
+      github.event.issue.pull_request != '' && 
+      (
+        contains(github.event.comment.body, '/rebase') || 
+        contains(github.event.comment.body, '/autosquash')
+      ) &&
+      (
+        github.event.comment.author_association == 'MEMBER' ||
+        github.event.comment.author_association == 'COLLABORATOR'
+      )
+    steps:
+      - uses: tibdex/github-app-token@v1
+        id: generate-token
+        with:
+          app_id: ${{ secrets.SG_REPO_GH_APP_ID }}
+          private_key: ${{ secrets.SG_REPO_GH_APP_PRIV_KEY }}
+      - name: Checkout the latest code
+        uses: actions/checkout@v3
+        with:
+          token: ${{ steps.generate-token.outputs.token }}
+          fetch-depth: 0 # otherwise, you will fail to push refs to dest repo
+      - name: Automatic Rebase
+        uses: cirrus-actions/[email protected]
+        with:
+          autosquash: ${{ contains(github.event.comment.body, '/autosquash') || contains(github.event.comment.body, '/rebase-autosquash') }}
+        env:
+          GITHUB_TOKEN: ${{ steps.generate-token.outputs.token }}