Add a Github Actions workflow for automatic rebase

Add /rebase to a comment to trigger an automatic rebase.

.github/workflows/rebase.yml

@@ -0,0 +1,22 @@
+on:
+  issue_comment:
+    types: [created]
+name: Rebase automatique
+jobs:
+  rebase:
+    name: Rebase
+    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase')
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Automatic Rebase
+      uses: cirrus-actions/rebase@master
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  # https://github.community/t5/GitHub-Actions/Workflow-is-failing-if-no-job-can-be-ran-due-to-condition/m-p/38186#M3250
+  always_job:
+    name: Aways run job
+    runs-on: ubuntu-latest
+    steps:
+      - name: Always run
+        run: echo "This job is used to prevent the workflow to fail when all other jobs are skipped."