Add workflow for automatically rebasing open PRs (#2612)

Introduce a workflow that's reponsible for automatically rebasing open
PRs when a commit has landed in the default (e.g. master) branch. Ensure
a comment is posted on that PR when the rebasing action job has failed.

Signed-off-by: timflannagan <timflannagan@gmail.com>

Author: timflannagan

.github/workflows/rebase.yaml

@@ -0,0 +1,20 @@
+name: rebase pull request
+on:
+  push:
+    branches:
+    - master
+jobs:
+  rebase:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: peter-evans/rebase@v1
+        with:
+          base: master
+      - name: Send failure message
+        uses: peter-evans/create-or-update-comment@v1
+        if: ${{ failure() }}
+        with:
+          issue-number: ${{ github.event.issue.number || github.event.number }}
+          body: |
+            Automatic rebasing for this PR has failed. You'll need to manually
+            rebase against the default branch.