bug#16818: Acknowledgement (Undo in region after markers in undo history relocated)

[Barry OReilly]

Another question: what is the purpose of undo-adjusted-markers?
simple.el is the only file I found that references it, and it's only
setting the variable without using it.

I found the specialized GC treatment of undo history markers in
compact_undo_list, so merely having references to the markers via
undo-adjusted-markers might be a motivation. But I can't think of a
particular reason for that.