Memoize repository storage shard in GroupWiki

This caused a bug where sometimes one shard was used for the repository
creation in `Wiki#create_wiki_repository`, but then a different one was
persisted to the DB in `GroupWiki#create_wiki_repository`.

So subsequent requests would use the wrong shard, and run into
`Gitlab::Git::Repository::NoRepository` errors.

ee/app/models/group_wiki.rb

@@ -19,7 +19,9 @@ class GroupWiki < Wiki
 
   override :repository_storage
   def repository_storage
-    container.group_wiki_repository&.shard_name || self.class.pick_repository_storage
+    strong_memoize(:repository_storage) do
+      container.group_wiki_repository&.shard_name || self.class.pick_repository_storage
+    end
   end
 
   override :hashed_storage?

ee/spec/models/group_wiki_spec.rb

@@ -70,6 +70,21 @@ RSpec.describe GroupWiki do
         it 'returns the default shard' do
           expect(subject.repository_storage).to eq('default')
         end
+
+        context 'when multiple shards are configured' do
+          let(:shards) { (1..).each }
+
+          before do
+            # Force pick_repository_storage to always return a different value
+            allow(Gitlab::CurrentSettings).to receive(:pick_repository_storage) { "storage-#{shards.next}" }
+          end
+
+          it 'always returns the same shard when called repeatedly' do
+            shard = subject.repository_storage
+
+            expect(subject.repository_storage).to eq(shard)
+          end
+        end
       end
 
       context 'when a tracking entry exists' do