undo doesn't fully undo -- won't restore original file.

I edited a file to test with an app that, oddly, didn't have an "eol" at the
end of the last file line.

My edit didn't work so I pressed "u" to undo and rewrote the original file.

But now it wasn't. It had an extra character at the end of the file (which
the app took as file corruption).

How can I get vim not to force an "eol" onto a file that didn't
originally have
one?

(Ideally, I'd like it to preserve state... not add them to files that
don't have them
and not delete them from those that do).

How do I get Vim to restore the file to the original state?

Thanks!

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