I updated the HowTo with a NOTE box that explains this behavior and how to proceed: Commit c81293d6.
The Docs page should be updated in <1 hour. Feel free to let me know if this NOTE box reflects your needs.
I have not yet had time to check how the UI behaves if the branch already exists but does not contain all commits. I will check later and adjust the box to contain that information as well. Feel free to let me know if someone knows already how GitLab behaves in such cases.