Add a `:repository` trait to the `:empty_project` factory

Right now this is a no-op. We're going to add it as a trait argument to
all existing usages of the `:project` factory, so that later we can flip
`:empty_project` to just be `:project`, forcing developers to opt-in to
the slower option when their tests truly require it.

spec/factories/projects.rb

@@ -49,6 +49,10 @@ FactoryGirl.define do
       request_access_enabled true
     end
 
+    trait :repository do
+      # no-op... for now!
+    end
+
     trait :empty_repo do
       after(:create) do |project|
         project.create_repository