Do not rewrite reference if already a cross reference

lib/gitlab/gfm/reference_rewriter.rb

@@ -49,12 +49,14 @@ module Gitlab
       def unfold_reference(reference, match, target_project)
         before = @text[0...match.begin(0)]
         after = @text[match.end(0)..-1]
-        referable = find_referable(reference)
 
+        referable = find_referable(reference)
         return reference unless referable
+
         cross_reference = referable.to_reference(target_project)
-        new_text = before + cross_reference + after
+        return reference if reference == cross_reference
 
+        new_text = before + cross_reference + after
         substitution_valid?(new_text) ? cross_reference : reference
       end