say when undo reaches last save

[Dan Jacobson]

Idea: you know how we usually do edit, save, edit, save ...
Well, when hitting the undo button, C-/, it would be nice to know when
we have reached at least the most recent previous save depth!

Sure, if we haven't saved recently, then we just hit undo until we see
the ** change into a -- in the modeline.

But, let's say we are lest happy with this compile of the file and
want to backtrack to the previous version just a few steps ago, and no
further.  Well, just hitting undo, there is just those "Undo!"
messages in the mimbuffer, no hint of when we pass the undo that
marked the last save.

No voices from the peanut gallery saying "just save a .bak", please.
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