change tracking quite unusable in LyX 1.6.x

[Uwe Stöhr]

Today me was sent a larger file created with LyX 1.5.x with change tracking enabled. When opening 
that file with LyX 1.6.1 every change is highlighted in blue, no matter if it was a deletion or 
addition. This make change tracking rather unusable.
In LyX 1.5.7 deletions are correctly highlighted red, additions blue.
I thought fixing this regression bug is easy but failed and don't have the time now to investigate 
further. Can someone else please have a look?

thanks and regards
Uwe