bug#14824: 24.3.50; Regression: problem with undo in Mail mode

[Glenn Morris]

Glenn Morris wrote:

> As another data point, the problem only happens with buffers that are
> not visiting files.

I think the issue might be:

buffer-undo-list has an element of the form:

 (t 0 0 0 -2000)

I don't know where -2000 picosecs comes from.

In undo.c, the test was:

  EMACS_TIME_NE (mod_time, base_buffer->modtime)

Now the test is:

(equal time (visited-file-modtime))

But visited-file-modtime returns 0 rather than -2000 picoseconds.