First, uninstall any git you already have (or thought you had)
sudo apt-get remove git
Then, get the latest git
sudo aptitude build-dep git-core
wget http://git-core.googlecode.com/files/git-1.7.9.1.tar.gz
tar xvzf git-1.7.9.1.tar.gz
cd git-1.7.9.1
./configure
make
sudo make install
cd ..
rm -r git-1.7.9.1 git-1.7.9.1.tar.gz
Next, you need Java Runtime Environment 7 which you can download from hereand then follow the installation instructions here
Download SmartGit from here, then
tar xvzf smartgit-generic-2_1_7.tar.gz
cd smartgit-2_1_7
cd bin
Then edit the smartgit shell script
gedit smartgit.sh
updating SMARTGIT_JAVA_HOME to where JRE is installed (e.g. /usr/lib/jvm/jrel1.7.0_03).
Now you're ready to run
./smartgit.sh
As part of the installation you will be prompted for the location of git, which you can find out by issuing in another terminal:
whereis git