Flush emptiness caches whenever needed

This ensures that the emptiness cache (used for Repository#empty? and
Repository#has_visible_content?) is flushed after comitting changes
(using the web editor, API or Git) for new repositories. Once a
repository is no longer empty there's no need to explicitly flush the
cache for Repository#empty?. The cache for
Repository#has_visible_content? in turn is already flushed whenever
needed.

Fixes gitlab-org/gitlab-ce#13387

app/models/repository.rb

@@ -239,6 +239,10 @@ class Repository
     end
 
     expire_branch_cache(branch_name)
+
+    # This ensures this particular cache is flushed after the first commit to a
+    # new repository.
+    expire_emptiness_caches if empty?
   end
 
   # Expires _all_ caches, including those that would normally only be expired

spec/models/repository_spec.rb

@@ -361,6 +361,20 @@ describe Repository, models: true do
 
       repository.expire_cache('master')
     end
+
+    it 'expires the emptiness cache for an empty repository' do
+      expect(repository).to receive(:empty?).and_return(true)
+      expect(repository).to receive(:expire_emptiness_caches)
+
+      repository.expire_cache
+    end
+
+    it 'does not expire the emptiness cache for a non-empty repository' do
+      expect(repository).to receive(:empty?).and_return(false)
+      expect(repository).to_not receive(:expire_emptiness_caches)
+
+      repository.expire_cache
+    end
   end
 
   describe '#expire_root_ref_cache' do