completion: Fix zsh parsing $GIT_PS1_SHOWUPSTREAM

Since GIT_PS1_SHOWUPSTREAM is a variable with space separated values and
zsh for loops do no split by space by default. The `-d' '` is a hacky
solution that works in both bash and zsh. The correct way to do that in
zsh would be do use read -rA and loop over the resulting array but -A is
not defined in bash.

Signed-off-by: Thomas Queiroz <thomasqueirozb@gmail.com>

contrib/completion/git-prompt.sh

@@ -141,14 +141,14 @@ __git_ps1_show_upstream ()
 
 	# parse configuration values
 	local option
-	for option in ${GIT_PS1_SHOWUPSTREAM-}; do
+	while read -r -d' ' option; do
 		case "$option" in
 		git|svn) upstream_type="$option" ;;
 		verbose) verbose=1 ;;
 		legacy)  legacy=1  ;;
 		name)    name=1 ;;
 		esac
-	done
+	done <<< "${GIT_PS1_SHOWUPSTREAM-} "
 
 	# Find our upstream type
 	case "$upstream_type" in