RFR: 2045: Make maintainer approval feature compatible with dependent pr feature. [v3]

[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.

Zhao Song has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Merge branch 'master' into SKARA-2045
   
   # Conflicts:
   #	bots/pr/src/main/java/org/openjdk/skara/bots/pr/ApprovalCommand.java
 - update
 - update
 - review comment
 - 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=02
  Stats: 85 lines in 5 files changed: 75 ins; 1 del; 9 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