Change `option` in `__git_ps1_show_upstream` to a local variable

The `option` variable in the `__git_ps1_show_upstream` function in
`contrib/completion/git-prompt.sh` [1] is not a local variable. Can it
be changed to a local variable like the other variables in
`__git_ps1_show_upstream`?

[1] 
https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh#L131