You will need to have a script for that. Unix/git-bash are already have this
script.
Read about it here: https://git-scm.com/book/en/v1/Git-Basics-Tips-and-Tricks
https://git-scm.com/book/en/v1/Git-Basics-Tips-and-Tricks#Auto-Completion
You have many options on how to install it. Simply choose the right way for you.,
https://github.com/bobthecow/git-flow-completion/wiki/Install-Bash-git-completion
OpenBSD Script
Here is a link to the OpenBSD script which you can use.
The file has header with all the instructions how to do it.
http://www.ualberta.ca/dept/aict/uts/software/openbsd/ports/4.6/i386/obj/git-1.7.0/git-1.7.0/contrib/completion/git-completion.bash
The above script instructions:
#!bash
#
# bash completion support for core Git.
#
# Copyright (C) 2006,2007 Shawn O. Pearce <spearce@spearce.org>
# Conceptually based on gitcompletion (http://gitweb.hawaga.org.uk/).
# Distributed under the GNU General Public License, version 2.0.
#
# The contained completion routines provide support for completing:
#
# *) local and remote branch names
# *) local and remote tag names
# *) .git/remotes file names
# *) git 'subcommands'
# *) tree paths within 'ref:path/to/file' expressions
# *) common --long-options
#
# To use these routines:
#
# 1) Copy this file to somewhere (e.g. ~/.git-completion.sh).
# 2) Added the following line to your .bashrc:
# source ~/.git-completion.sh
#
# 3) Consider changing your PS1 to also show the current branch:
# PS1='[\u@\h \W$(__git_ps1 " (%s)")]\$ '
#
# The argument to __git_ps1 will be displayed only if you
# are currently in a git repository. The %s token will be
# the name of the current branch.
#
# In addition, if you set GIT_PS1_SHOWDIRTYSTATE to a nonempty
# value, unstaged (*) and staged (+) changes will be shown next
# to the branch name. You can configure this per-repository
# with the bash.showDirtyState variable, which defaults to true
# once GIT_PS1_SHOWDIRTYSTATE is enabled.
#
# You can also see if currently something is stashed, by setting
# GIT_PS1_SHOWSTASHSTATE to a nonempty value. If something is stashed,
# then a '$' will be shown next to the branch name.
#
# If you would like to see if there're untracked files, then you can
# set GIT_PS1_SHOWUNTRACKEDFILES to a nonempty value. If there're
# untracked files, then a '%' will be shown next to the branch name.