MergeRequests::PushOptionsHandlerService has been updated to allow
creating and updating merge requests with the `title` and
`description` set using git push options.

To create a new merge request and set its title and description:

      git push -u origin -o merge_request.create \
        -o merge_request.title="My title" \
        -o merge_request.description="My description"

To update an existing merge request and set its title and
description:

      git push -u origin -o merge_request.title="My title" \
        -o merge_request.description="My description"

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/64320