sh: simplified custom function __git_ps1 to display current branch or tag name in git repo

__git_ps1.sh

#!/bin/sh
############ *posix* #############
# shellcheck shell=sh enable=all #
##################################
#
# If one is missing the original *__git_ps1* function from @git/git package, this wrapper can be utilized as
# temporary solution
#
# @see https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh
#
# Just like the original, it delivers PS1 prompt in both ways thru export or output to stdout, supplied either with
# a current branch name like *main*, *dev* etc. or tags like  *v1.4.0* or with nothing, if current working directory
# is outside of any git tree
#
# Itʼs a pity this implementation can offer no colorized PS1 output like *__git_ps1* function from @git/git package
# does, but thatʼs ok for less than 30 lines of code vs over 500 lines of original
#
# Designed and developed by Juliy V. Chirkov https://juliyvchirkov.dev at 2021-2023 under the MIT lisense
# https://t.me/juliyvchirkov
#
###################################################################################################################
#
# At 2023 Iʼve totally turned away in daily shell scripting from *bash* in favor of fully *posix* compliant way of 
# implementation focused on sh, the standard command language interpreter
#
# @see https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html
#
# The thing is after the pros and cons has been completely reviewed, Iʼve found it quite reasonable to reject minor
# amenities *bash* delivers to simplify a coding routine for the sake of portability to develop things that are 
# a priori blessed to success on any device regardless of a shell. *bash*, *dash*, *ksh*, *posh*, *yash*, *zsh*, 
# whatever - a code will run everywhere the same way as long as a shell honours *posix* standards
#
# Long story short, during summer and fall of 2023 Iʼve revised and reimplemented from scratch the lionʼs share of
# own shell scripts, and this wrapper has got its code upgrade among others

[ -z "${SHDEBUG}" ] || set -vx

__git_ps1() {
    __git_ps1Template="${3:-${1:- %s}}"
    __git_ps1Branch="$(/usr/bin/git symbolic-ref -q --short HEAD 2>/dev/null)"

    if [ -z "${__git_ps1Branch}" ]; then
        __git_ps1Branch="$(/usr/bin/git describe --tags --exact-match 2>/dev/null)"

        [ -z "${__git_ps1Branch}" ] || __git_ps1Branch="(${__git_ps1Branch})"
    fi

    if [ -n "${__git_ps1Branch}" ]; then
        if [ "${__git_ps1Template%\%s*}" = "${__git_ps1Template}" ]; then
            __git_ps1Template="${__git_ps1Template} %s"
        fi

        if [ $# -lt 2 ]; then
            #############################
            # shellcheck disable=SC2059 #
            #############################
            command printf -- "${__git_ps1Template}" "${__git_ps1Branch}"
        else
            #############################
            # shellcheck disable=SC2059 #
            #############################
            PS1="${1}$(command printf -- "${__git_ps1Template}" "${__git_ps1Branch}")${2}"

            export PS1 >/dev/null 2>&1
        fi
    fi

    unset __git_ps1Template __git_ps1Branch
}