To install git 1.8.0 you need to compile the source code.
You need to clone the git repository (git clone https://github.com/git/git.git), enter the git directory, change the current version to 1.8.0 or other version that you want (git checkout v.1.8.0) and type the following commands:
make configure
./configure --prefix=/usr/local
make
make install
It will install git under /usr/local directory. For more information about git installation, like documentation, you can take a look in INSTALL file localized in git repository root.
