Merge branch 'ee-parity' into 'master'

Add method missing from EE

## Why was this MR needed?

`ProjectWiki#repository_exists?` is present in EE but not on CE, and there's no particular reason why it shouldn't

See merge request !5802

app/models/project_wiki.rb

@@ -56,6 +56,10 @@ class ProjectWiki
     end
   end
 
+  def repository_exists?
+    !!repository.exists?
+  end
+
   def empty?
     pages.empty?
   end