Refactor IssuableBaseService label processing

In issuable_base_service.rb the code processing labels is making
the update() method not as clear as it could be. Let's refactor
this code into a new assign_requested_labels() method.

While at it, lets rename labels_changing? into ids_changing?
as the method is not specific to labels and could be used with
other arrays of ids.

app/services/issuable_base_service.rb

@@ -190,11 +190,7 @@ class IssuableBaseService < BaseService
     change_additional_attributes(issuable)
     old_associations = associations_before_update(issuable)
 
-    label_ids = process_label_ids(params, existing_label_ids: issuable.label_ids)
-    if labels_changing?(issuable.label_ids, label_ids)
-      params[:label_ids] = label_ids
-      issuable.touch
-    end
+    assign_requested_labels(issuable)
 
     if issuable.changed? || params.present?
       issuable.assign_attributes(params)
@@ -297,10 +293,6 @@ class IssuableBaseService < BaseService
     update_task(issuable)
   end
 
-  def labels_changing?(old_label_ids, new_label_ids)
-    old_label_ids.sort != new_label_ids.sort
-  end
-
   def has_title_or_description_changed?(issuable)
     issuable.title_changed? || issuable.description_changed?
   end
@@ -349,6 +341,20 @@ class IssuableBaseService < BaseService
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
+  def assign_requested_labels(issuable)
+    label_ids = process_label_ids(params, existing_label_ids: issuable.label_ids)
+    return unless ids_changing?(issuable.label_ids, label_ids)
+
+    params[:label_ids] = label_ids
+    issuable.touch
+  end
+
+  # Arrays of ids are used, but we should really use sets of ids, so
+  # let's have an helper to properly check if some ids are changing
+  def ids_changing?(old_array, new_array)
+    old_array.sort != new_array.sort
+  end
+
   def toggle_award(issuable)
     award = params.delete(:emoji_award)
     AwardEmojis::ToggleService.new(issuable, award, current_user).execute if award