The problem with using an external editor is we don't have access to the previous master template to actually compare what the changes were between the previous master and current master (it would be required for automatic template merging to know what the difference between the two masters were before you compare the diff between the customized one).
Personally, I'd prefer an automatic diff/template merge system as well.
Plus a generalized diff class would be useful for other things (like comparing post edit history if we ever get post edit history).