Thank you very much for your answer! 
I’ll try to raise a bug request on their site; and yes I’m using this meld you referred to.
Unfortunately, sudo diff is not practical to me. I need meld as a simple graphical tool to quickly compare and eventually merge .pacnew - files. It is the only thing where I couldn’t find a a good non-graphical alternative.
On the bright side, I use it solely for that indeed. 
As for now, I switched to do this instead:
pacdiff -o
sudo -i
meld /etc/thisandthatfile /etc/thisandthatfile.pacnew
exit