clear variables with `unset` instead of a subshell

[LucasLarson]

instead of

(
  a=b
)

use instead

a=b
unset a

gunstage/bin/git-unstage

Lines 21 to 57 in a420181

	(
	# check whether we’re in a Git repository
	# https://stackoverflow.com/a/53809163
	if git rev-parse --is-inside-work-tree >/dev/null 2>&1; then
	# run the same command against each argument, if any
	# otherwise use `.` for everything in the current directory and below
	for file in "${@:-.}"; do
	# https://github.com/gggritso/gggritso.com/commit/a07b620
	git reset --quiet HEAD -- "${file}"
	# perform a `git status` only if the loop was successful
	done && git status
	else
	# we’re not in a Git repository
	# store the failed if as a return status
	# https://github.com/ohmyzsh/ohmyzsh/pull/9238#discussion_r484806772
	gunstage=$?
	# if called from outside a Git repository, then
	# provide instructions on how to use it
	printf 'gunstage must be called from within a Git repository\n'
	printf 'create a file and a repository, then stage and gunstage:\n
	touch file && # create a file called '
	printf '\xe2\x80\x9cfile\xe2\x80\x9d \x5c
	git init && # make a Git repository if necessary \x5c
	git add file && git status # stage the file you just created
	# now use \x60gunstage\x60 to unstage the file
	gunstage file # huzzah.\n\n
	gunstage # \xf0\x9f\x94\xab \x60git unstage\x60 as a service
	https://github.com/LucasLarson/gunstage\n\n'
	return "${gunstage}"
	fi
	)