How to merge?
git fetch @@url@@ refs/mr/rg_xxxxxxxx
git merge ???

How to "see" all merge requests as branches:
Add, in config file, under the remote you want, a line like this:
fetch = +refs/namespaces/*:refs/remotes/your_remote_name_for_example_origin/mr/*
After you run a git fetch, you will have all the merge requests locally.
You can do ???