Manual:$wgGitBin/ja

詳細
Absolute path to the git binary.

この変数は、git でチェックアウトしたものを直接実行している場合のみ使用されます. 大部分の利用者はこのようにしていないため、git を利用できなくても気にしないでください. If the git binary is not present at the specified location, you will not be able to see git hashes on Special:Version (and you will see a PHP warning). You can either install git in that location and adjust  accordingly or set   to avoid the warning.

If you however do run a git checkout (meaning you have a  folder in your installation directory), but Git is not available, then you should set. Otherwise you will get errors on top of the "Special:Version" page telling you that the according binary would not have been found.