Fix nil user handling in UpdateMirrorService
Previously an exception was hit quite frequently:
NoMethodError: undefined method `can?' for nil:NilClass
app/services/projects/update_mirror_service.rb:9:in `execute'
unless current_user.can?(:push_code_to_protected_branches, project)
lib/gitlab/metrics/instrumentation.rb:153:in `execute'
retval = super
app/workers/repository_update_mirror_worker.rb:13:in `perform'
result = Projects::UpdateMirrorService.new(@project, @current_user).execute
lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
yield
lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
yield
...
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Closes #605 (closed)