Cache the value of safe_message

Also, just for extra paranoia, only call safe_message once in the
decorator methods

Adds specs to make sure it still works

app/models/commit.rb

@@ -107,7 +107,7 @@ class Commit
   end
 
   def safe_message
-    utf8 message
+    @safe_message ||= utf8 message
   end
 
   def created_at