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

[Glenn Morris]

>> A shot in the dark: it might be due to the finer timestamps (since
>> these were introduced fairly recently, whereas Aaron's code is actually
>> pretty old (even tho it was installed only recently)).

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

> I am a little surprised this was broken in trunk for the last 6
> months, and nobody had noticed. 

I'm not that surprised.

> The automated test suite could catch this by adding a line containing
> (should (not (buffer-modified-p))) to one of the unit tests.

Yep.