RFR: 2045: Make maintainer approval feature compatible with dependent pr feature.

[Zhao Song]

A user reported that a dependent pr in a repo which configured with maintainer approval feature is not marked for approval. 

After investigation, I realized that the skara bot would determine whether this pr needs maintainer approval by checking whether merging into the target branch needs maintainer approval. 
  
In this case, we only configured that merging into master branch of jdk21u needs maintainer approval, however, for dependent pull requests, the target branch is pr/XXX. 

To fix this issue, we should let the skara bot be able to find the real target ref.

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Commit messages:
 - fix an issue
 - add test
 - SKARA-2045

Changes: https://git.openjdk.org/skara/pull/1562/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1562&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2045
  Stats: 84 lines in 4 files changed: 75 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/skara/pull/1562.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1562/head:pull/1562

PR: https://git.openjdk.org/skara/pull/1562