git-prompt.sh: Don't error on null ${ZSH,BASH}_VERSION, $short_sha

When the shell is in "nounset" or "set -u" mode, referencing unset or
null variables results in an error. Protect $ZSH_VERSION and
$BASH_VERSION against that, and initialize $short_sha before use.

Signed-off-by: Ville Skyttä <ville.skytta@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -355,8 +355,8 @@ __git_ps1 ()
 	# incorrect.)
 	#
 	local ps1_expanded=yes
-	[ -z "$ZSH_VERSION" ] || [[ -o PROMPT_SUBST ]] || ps1_expanded=no
-	[ -z "$BASH_VERSION" ] || shopt -q promptvars || ps1_expanded=no
+	[ -z "${ZSH_VERSION-}" ] || [[ -o PROMPT_SUBST ]] || ps1_expanded=no
+	[ -z "${BASH_VERSION-}" ] || shopt -q promptvars || ps1_expanded=no
 
 	local repo_info rev_parse_exit_code
 	repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
@@ -368,7 +368,7 @@ __git_ps1 ()
 		return $exit
 	fi
 
-	local short_sha
+	local short_sha=""
 	if [ "$rev_parse_exit_code" = "0" ]; then
 		short_sha="${repo_info##*$'\n'}"
 		repo_info="${repo_info%$'\n'*}"