Refactor params processing in BuildService

In build_service.rb, let's refactor some param processing code
and comments from the execute() method into a new
filter_label_id_params() method.

While at it let's also add a few comments.

app/services/merge_requests/build_service.rb

@@ -19,22 +19,14 @@ module MergeRequests
       # Force remove the source branch?
       merge_request.merge_params['force_remove_source_branch'] = force_remove_source_branch
 
+      # Only assign merge requests params that are allowed
       self.params = assign_allowed_merge_params(merge_request, params)
 
+      # Filter out params that are either not allowed or invalid
       filter_params(merge_request)
 
-      # merge_request.assign_attributes(...) below is a Rails
-      # method that only work if all the params it is passed have
-      # corresponding fields in the database. As there are no fields
-      # in the database for :add_label_ids and :remove_label_ids, we
-      # need to remove them from the params before the call to
-      # merge_request.assign_attributes(...)
-      #
-      # IssuableBaseService#process_label_ids takes care
-      # of the removal.
-      params[:label_ids] = process_label_ids(params, extra_label_ids: merge_request.label_ids.to_a)
-
-      merge_request.assign_attributes(params.to_h.compact)
+      # Filter out :add_label_ids and :remove_label_ids params
+      filter_label_id_params
 
       merge_request.compare_commits = []
       set_merge_request_target_branch
@@ -77,6 +69,21 @@ module MergeRequests
       end
     end
 
+    def filter_label_id_params
+      # merge_request.assign_attributes(...) below is a Rails
+      # method that only work if all the params it is passed have
+      # corresponding fields in the database. As there are no fields
+      # in the database for :add_label_ids and :remove_label_ids, we
+      # need to remove them from the params before the call to
+      # merge_request.assign_attributes(...)
+      #
+      # IssuableBaseService#process_label_ids takes care
+      # of the removal.
+      params[:label_ids] = process_label_ids(params, extra_label_ids: merge_request.label_ids.to_a)
+
+      merge_request.assign_attributes(params.to_h.compact)
+    end
+
     def find_source_project
       source_project = project_from_params(:source_project)
       return source_project if source_project.present? && can?(current_user, :create_merge_request_from, source_project)