I was digging through some old bugs, in ran into this one.

> It's still significantly coarser than using diff: revert-buffer is
> treated as a single delete+insert, so only the markers before the first
> modification or after the last modification are preserved.

What does M-x diff-buffer-with-file not do what you are trying to
achieve?
Also M-x ediff-current-file might be usable.


-- Toon