Vim: Maintain the history of a file after it has been changed and reloaded

Question

If I'm editing a file in Vim, then some external program changes the file, Vim tells me:

W11: Warning: File "test.erl" has changed since editing started
See ":help W11" for more info.
[O]K, (L)oad File: 

If I Load the file, though, all the undo history is lost.

Is there any way to avoid that?

Answer 1

Update: it appears that this is one of the new features in Vim 7.3: https://groups.google.com/group/vim_announce/browse_thread/thread/66c02efd1523554b

Answer 2

I don't believe this is possible.

There is a very useful patch available for the vim source code available here that keeps the undo history after exiting vim and restarting. However, I've just tried it and it seems to fail when the file is edited externally. It might be worth contacting the author or reporting a bug on the patch website to see if this can be overcome.