Broken Git installation on my Mac. How do I fix it?
To debug this, start by executing:

type -a git

and

echo $PATH

In OP's case, running the above commands gave the following output respectively:

git is /usr/bin/git
git is /usr/local/bin/git


and

/usr/local/opt/php@7.0/sbin
/usr/local/opt/php@7.0/bin
/Applications/MAMP/bin/php/php7.1.8/bin
/usr/bin
/bin
/usr/sbin
/sbin
/usr/local/bin
/usr/local/autoconf
/opt/X11/bin
/usr/local/share/dotnet
~/.dotnet/tools
/Library/Frameworks/Mono.framework/Versions/Current/Commands
/Applications/Xamarin Workbooks.app/Contents/SharedSupport/path-bin
/Applications/Android Studio.app/Contents/plugins/flutter/bin


(in the above output is formatted for readability by replacing : with newline)

Now, run the following commands (using absolute path) to check if git package is actually installed:

/usr/bin/git --version

and

/usr/local/bin/git --version

The above gave the following output, respectively:

active developer path does not exist


and

git version 2.21.0


This indicates that Homebrew installed copy is still present in the system. However, Xcode/system installed copy (which should take precedence due to $PATH variable configuration) wasn't accessible because developer path couldn't be found.

To resolve the issue and to use the Xcode/system installed copy of git, run:

xcode-select --reset

This will resolve the error.

To use the latest version of Git, install the same via Homebrew by running:

brew install git

and set the PATH variable so that /usr/local/bin takes precedence over /usr/bin.