git-prompt.sh: localize `option` in __git_ps1_show_upstream

The variable 'option' is used in __git_ps1_show_upstream()
without being localized.

This clobbers the variable the user may be using for other
purposes, which is bad.  Luckily, $option is not used to carry
information around in the script as a global variable.  The use
of it in this script has very limited scope (namely, only inside
this function), so just declare that it is "local".

Signed-off-by: Sibo Dong <sibo.dong@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -137,6 +137,7 @@ __git_ps1_show_upstream ()
 	done <<< "$output"
 
 	# parse configuration values
+	local option
 	for option in ${GIT_PS1_SHOWUPSTREAM}; do
 		case "$option" in
 		git|svn) upstream="$option" ;;