Fix spurious scores in comments

Fix spurious scores in comments

The comment popup is also used to display engine PV info, but when the
contents of the window is written back when we select a variation,
it was treated as a real comment. This is fixed now by checing in
ReplaceComment() if the comment starts with the PV info, and if so,
ignore the latter. This solves the problem, but I guess the real bug is
that selecting a variation tries to write the comment even if it did not
change.