[ci][backport-pr] Fetch all commits

Fixes a bug in which the target commit isn't checked out by default.

gherrit-pr-id: I8c3db36d20b511c31ec774db68896f2fe4b539bb

.github/workflows/backport-pr.yml

@@ -29,6 +29,10 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           ref: ${{ github.event.inputs.target_branch }}
+          # Check out the entire repository so that the target commit is checked
+          # out (by default, only fetches the single commit identified by the
+          # `ref` argument).
+          fetch-depth: 0
           persist-credentials: false
       - name: Cherry-pick commit
         run: |