The meld diff tool comes as standard with RabbitVCS. Unfortunately, you only get version 1.5.3 with Ubuntu 12.04 by default. Unfortunately, I have not found a PPA yet for meld (fingers crossed for the webupd8 team). For the time being, use:
wget http://ftp.uk.debian.org/debian/pool/main/m/meld/meld_1.8.4-1_all.deb sudo dpkg -i meld_1.8.4-1_all.deb rm meld_1.8.4-1_all.deb
This should upgrade you from 1.5.3 to 1.8.4.
