It appears that git18 isno longer available from RPMForge, at least in CentOS 6.6. Luckily, I came across the following instructions from a GitLabCentOS recipe:
Verify that the EPEL and PUIAS Computational repositories are enabled:
yumrepolist
Install Git 1.8
yum--enablerepo=PUIAS_6_computational install git
Note: I am not quite happy with this solution, as it only netted me Git 1.8.3 however I do not have the time to find a better solution right now.