How to update Git (actually remove apple git and install new one via brew)?
You can try to remove CLI tools alone, which is what comes with Apple git and that may solve your problem.

CLI Tools Removal


https://gist.github.com/yoneken/3284561


if that didn't do it, that means that CLI Tools were downloaded separate from Xcode and you can try this to remove it:

sudo rm -rf /Library/Developer/CommandLineTools


Once that's gone you just check to make sure it got removed and then reinstall it with Homebrew. Hope this may help.