Add gitium_git_bin_path filter

https://wordpress.org/support/topic/wrong-path-for-git/

This is greatly anticipated, another route would be to make it as a defined constant like the gitignore contents. Any way to override it is requested. I you are interested I can make a PR for the git path as a defined constant (our host has a git in /usr/bin which does not get updated, and have a recent version in an obscure sub directory to cpanel.