How to create a GitLab merge request via command line

It's not natively supported, but it's not hard to throw together. The gitlab API has support for opening MR: https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/merge_requests.md#create-mr

As of GitLab 11.10, if you're using git 2.10 or newer, you can automatically create a merge request from the command line like this:

git push -o merge_request.create

More information can be found in the docs.