Simplify Git interactions in AfterImportService

app/services/projects/after_import_service.rb

 module Projects
   class AfterImportService
-    RESERVED_REFS_REGEXP =
-      %r{\Arefs/(?:#{Regexp.union(*Repository::RESERVED_REFS_NAMES)})/}
+    RESERVED_REF_PREFIXES = Repository::RESERVED_REFS_NAMES.map { |n| File.join('refs', n, '/') }
     def initialize(project)
       @project = project
@@ -9,7 +8,7 @@ module Projects
     def execute
       Projects::HousekeepingService.new(@project).execute do
-        repository.delete_refs(*garbage_refs)
+        repository.delete_all_refs_except(RESERVED_REF_PREFIXES)
       end
     rescue Projects::HousekeepingService::LeaseTaken => e
       Rails.logger.info(
@@ -18,10 +17,6 @@ module Projects
     private
-    def garbage_refs
-      @garbage_refs ||= repository.all_ref_names_except(RESERVED_REFS_REGEXP)
-    end
-
     def repository
       @repository ||= @project.repository
     end

lib/gitlab/git/repository.rb

@@ -250,11 +250,17 @@ module Gitlab
         branch_names + tag_names
       end
+      def delete_all_refs_except(prefixes)
+        delete_refs(*all_ref_names_except(prefixes))
+      end
+
       # Returns an Array of all ref names, except when it's matching pattern
       #
       # regexp - The pattern for ref names we don't want
-      def all_ref_names_except(regexp)
-        rugged.references.reject { |ref| ref.name =~ regexp }.map(&:name)
+      def all_ref_names_except(prefixes)
+        rugged.references.reject do |ref|
+          prefixes.any? { |p| ref.name.start_with?(p) }
+        end.map(&:name)
       end
       # Discovers the default branch based on the repository's available branches