When a user updates a merge request coming from a fork, they should
not be able to set `force_remove_source_branch` if they cannot push
code to the source project.
Otherwise developers of the target project could remove the source
branch of the source project by setting this flag through the API.
We have one auto merge strategy today - Merge When Pipeline
Succeeds.
In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.
Removed arguments
Fix spec
