How to get git update-index working with macOS? I installed git with brew install git

Now I want to use the update-index parameter, but that doesn't work because git-update-index is not installed.

How can I install this via Homebrew or another way for macOS?

EDIT

I am running macOS High Sierra 10.13.6 with git version 2.18.0 (both currently at the time of this writing).

$ type -a git
git is /usr/local/bin/git
git is /usr/bin/git


I'll check that tomorrow on work, because with my home-mac it works the correct way.
You are possible executing the command incorrectly. There is no git-update-index command.

Instead try, git update-index.

There is no additional steps required to install git-update-index.