Add a `/create_merge_request` quick action
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15176 adds a /create_branch
quick action on issues, where you can optionally pass a custom branch name.
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13884 adds the ability to customise the branch name used when creating an MR from an issue.
What's missing? The intersection of the two: a /create_merge_request
quick action, with an optional argument for the branch name.