"Merge commit with semi-linear history" should propose "merge conflict" when Fast-forward merge is not possible

Summary

A repo with "Merge method" is "Merge commit with semi-linear history" should propose "merge conflict" when Fast-forward merge is not possible

Steps to reproduce

• Create a conflict on your MR.
• Set the repo config "Merge method" to "Merge commit" and you can see the usefull "resolve conflict"
• Set the repo config "Merge method" to "Merge commit with semi-linear history" and you have just "Fast-forward merge is not possible. To merge this request, first rebase locally."

What is the current bug behavior?

With "Merge commit with semi-linear history", when there is a conflict on the MR, the "resolve conflict" option is not proposed anymore while it's still proposed with "Merge commit" option

What is the expected correct behavior?

With "Merge commit with semi-linear history", when there is a conflict on the MR, the "resolve conflict" option should be proposed as done with "Merge commit" option

Output of checks

This bug happens on GitLab.com