fix the approvals_before_merge project fallback

the param :target_project_id is only sent when creating the MR.
upon an update, the @merge_request.target_project will be used.

ee/app/controllers/ee/projects/merge_requests/application_controller.rb

@@ -36,8 +36,13 @@ module EE
         def clamp_approvals_before_merge(mr_params)
           return mr_params unless mr_params[:approvals_before_merge]
 
-          target_project = @project.forked_from_project if @project.id.to_s != mr_params[:target_project_id]
-          target_project ||= @project
+          target_project = if @merge_request
+                             @merge_request.target_project
+                           elsif @project.id.to_s != mr_params[:target_project_id]
+                             @project.forked_from_project
+                           else
+                             @project
+                           end
 
           if mr_params[:approvals_before_merge].to_i <= target_project.approvals_before_merge
             mr_params[:approvals_before_merge] = nil