[GQL] All merge request permissions need gitaly

This dries up the code a little.

app/graphql/types/permission_types/merge_request.rb

@@ -3,6 +3,11 @@
 module Types
   module PermissionTypes
     class MergeRequest < BasePermissionType
+      PERMISSION_FIELDS = %i[push_to_source_branch
+                             remove_source_branch
+                             cherry_pick_on_current_merge_request
+                             revert_on_current_merge_request].freeze
+
       present_using MergeRequestPresenter
       description 'Check permissions for the current user on a merge request'
       graphql_name 'MergeRequestPermissions'
@@ -10,10 +15,9 @@ module Types
       abilities :read_merge_request, :admin_merge_request,
                 :update_merge_request, :create_note
 
-      permission_field :push_to_source_branch, method: :can_push_to_source_branch?, calls_gitaly: true
-      permission_field :remove_source_branch, method: :can_remove_source_branch?, calls_gitaly: true
-      permission_field :cherry_pick_on_current_merge_request, method: :can_cherry_pick_on_current_merge_request?
-      permission_field :revert_on_current_merge_request, method: :can_revert_on_current_merge_request?
+      PERMISSION_FIELDS.each do |field_name|
+        permission_field field_name, method: :"can_#{field_name}?", calls_gitaly: true
+      end
     end
   end
 end