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
...
(26 additional frame(s) were not displayed)
```

Closes #605

See merge request !434