Merge branch 'fix-nil-user-update-mirror-service' into 'master'

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

CHANGELOG-EE

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.9.0 (unreleased)
+  - Fix nil user handling in UpdateMirrorService
 
 v 8.8.3
   - Add standard web hook headers to Jenkins CI post. !374

app/services/projects/update_mirror_service.rb

@@ -6,7 +6,7 @@ module Projects
     def execute
       return false unless project.mirror?
 
-      unless current_user.can?(:push_code_to_protected_branches, project)
+      unless can?(current_user, :push_code_to_protected_branches, project)
         return error("The mirror user is not allowed to push code to all branches on this project.")
       end
 

spec/services/projects/update_mirror_service_spec.rb

@@ -75,6 +75,16 @@ describe Projects::UpdateMirrorService do
         expect(result[:status]).to eq(:error)
       end
     end
+
+    describe "when no user is present" do
+      let(:mirror_user) { }
+
+      it "fails" do
+        result = subject.execute
+
+        expect(result[:status]).to eq(:error)
+      end
+    end
   end
 
   def fetch_mirror(repository)