Fix 500 error when project is empty

lib/gitlab/markdown.rb

@@ -173,7 +173,7 @@ module Gitlab
     end
 
     def reference_commit(identifier)
-      if commit = @project.commit(identifier)
+      if @project.valid_repo? && commit = @project.commit(identifier)
         link_to(identifier, project_commit_path(@project, commit), html_options.merge(title: CommitDecorator.new(commit).link_title, class: "gfm gfm-commit #{html_options[:class]}"))
       end
     end