If you try to edit a file on a protected branch as a developer, the branch should default to patch-n
dev issue: https://dev.gitlab.org/gitlab/gitlabhq/issues/2242
If you are a developer, you are not able to commit to a protected branch.
If you still want to edit something in the UI, the branch should be set to patch-1 (if available, otherwise patch-2, etc), and you should be able to create a MR immeidately (#3059 (closed)).
cc @sytses