How to run apple and custom git side-by-side
I've talked about modifying system tools before and the short of it is: don't do it! Things will use the system tools expecting a certain version of a certain tool and if you change that you could break many things.

Instead, change your path. For example, if your git is located at /usr/local/git/bin/git, add the following to your ~/.bash_profile or equivalent:

export PATH=/usr/local/git/bin:$PATH


This way, you can use your git by running git but the system git will remain available.