Currently, ghprb checks every pull request on a repository. I need a way to specify a specific branch since the "Branch Specifier" and "Refspec" option are used by ghprb.

The pull requests are not grouped by branch in the refs directory, so there is no way to do it with a custom Refspec that I can see.

The plugin already checks the branch here: https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbPullRequest.java#L38

It would make sense to have a check right after that line to see if there is a defined allowed branch and if it matches that.