Allow retry in get-merged-pr (#1427)

This should fix #1426. I didn't
test it, but the retry pattern is the same as in
https://github.com/mmtk/mmtk-core/blob/76ef95a3da8b1f2f1420bfb71122ac37c77761b5/.github/workflows/auto-merge-inner.yml#L98-L103

Author: qinsoon

.github/workflows/auto-merge.yml

@@ -16,19 +16,32 @@ jobs:
       pr: ${{ steps.get-pr.outputs.pr }}
     steps:
       - id: get-commit-hash
-        run: echo "commit=${GITHUB_SHA}" >> "$GITHUB_OUTPUT"
-      - name: Check commit hash and PR
         run: |
+          echo "commit=${GITHUB_SHA}" >> "$GITHUB_OUTPUT"
           echo "Commit hash: ${GITHUB_SHA}"
-          echo "Pull request for the commit is below:"
-          gh pr list --search "${GITHUB_SHA}" --state merged --repo mmtk/mmtk-core
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - id: get-pr
         run: |
-          PR_NUMBER=$(gh pr list --search "${GITHUB_SHA}" --state merged --repo mmtk/mmtk-core --json number --jq '.[0].number')
-          echo "pr=$PR_NUMBER" >> "$GITHUB_OUTPUT"
-          echo "Output pr=$PR_NUMBER"
+          for n in {1..5}; do
+              echo "Wait 30s then try get the pull request number"
+              sleep 30
+
+              echo "Pull request for the commit is below:"
+              gh pr list --search "${GITHUB_SHA}" --state merged --repo mmtk/mmtk-core
+              echo "Pull request for the commit is above."
+
+              PR_NUMBER=$(gh pr list --search "${GITHUB_SHA}" --state merged --repo mmtk/mmtk-core --json number --jq '.[0].number')
+
+              if [ -n "$PR_NUMBER" ] && [ "$PR_NUMBER" != "null" ]; then
+                echo "Found PR: $PR_NUMBER"
+                echo "pr=$PR_NUMBER" >> "$GITHUB_OUTPUT"
+                echo "Output pr = $PR_NUMBER"
+                exit 0
+              fi
+
+              echo "PR not found, retrying..."
+          done
+          echo "We can't find the merged PR for ${GITHUB_SHA}"
+          exit 1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}