I'd like to propose a command 'ediff-undo' that compares the current
buffer with how it would look after undo.  It supports a prefix arg (to
specify multiple undo steps) and respects the active region (to restrict
undo operations to that region).

Some less obvious situations where I've found this helpful:

- When a buffer has been modified by an external program or a
  complicated Emacs command.

- After replacing part of a buffer with text obtained externally (e.g.,
  copied from an external email client).

- For a quick recap of recent edits to a buffer or region, via something
  like C-u C-u C-u M-x ediff-undo.

The implementation is a straightforward combination of parts of
'ediff-current-file' and 'undo'.  Any feedback would be welcome.