A user reported that a dependent pr in a repo which configured with maintainer approval feature is not marked for approval.
https://github.com/openjdk/jdk21u/pull/200

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.