Step 1. Fetch and check out the branch for this merge request

git fetch origin
git checkout -b "add_mutex" "origin/add_mutex"

Step 2. Review the changes locally

Step 3. Merge the branch and fix any conflicts that come up

git fetch origin
git checkout "origin/master"
git merge --no-ff "add_mutex"

Step 4. Push the result of the merge to GitLab

git push origin "master"

Note that pushing to GitLab requires write access to this repository.

Tip: You can also checkout merge requests locally by following these guidelines.