RFR: 2051: Use real target ref to determine whether this pr needs maintainer approval in CheckWorkItem

[Zhao Song]

After deployed [SKARA-2045](https://bugs.openjdk.org/browse/SKARA-2045) to production, I found that some dependent pull requests are still not updated. Then I realized that in CheckWorkItem, we also need to use real target ref to determine whether this pr needs maintainer approval.

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Commit messages:
 - SKARA-2051

Changes: https://git.openjdk.org/skara/pull/1566/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1566&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2051
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1566.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1566/head:pull/1566

PR: https://git.openjdk.org/skara/pull/1566