Avoid Push CI failing to report due to many commits being merged (#19496)

* Change the depth to 20

* Add comment
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

.github/workflows/self-push.yml

@@ -526,6 +526,11 @@ jobs:
           echo "env.CI_SHA = ${{ env.CI_SHA }}"
 
       - uses: actions/checkout@v2
+        # To avoid failure when multiple commits are merged into `main` in a short period of time.
+        # Checking out to an old commit beyond the fetch depth will get an error `fatal: reference is not a tree: ...
+        # (Only required for `workflow_run` event, where we get the latest HEAD on `main` instead of the event commit)
+        with:
+          fetch-depth: 20
 
       - name: Update clone using environment variables
         run: |