How to pull only one merge-request/ branch

I'm configuring MergeRequest build with **Build when a change is pushed to GitLab**. according to [docs](https://github.com/jenkinsci/gitlab-plugin#freestyle-jobs)

> In the Advanced settings, set Name to origin and Refspec to +refs/heads/*:refs/remotes/origin/* +refs/merge-requests/*/head:refs/remotes/origin/merge-requests/*

But this results in ALL merge-request/* branches being fetched, which in my case takes a lot of time as there are thousands of them. But really there is a need for only two, namely **master** and my **merge-request/42** branches. So my question is how can I change the Refspec to pull only these branches instead of all? Or any other change I can make to speed up the process.

Thank you in advance!