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>