completion: protect prompt against unset SHOWUPSTREAM in nounset mode

As it stands, the only call site of `__git_ps1_show_upstream` checks
that the `GIT_PS1_SHOWUPSTREAM` variable is set, so this is effectively
a no-op. However, that might change, and chances of noticing the
unprotected use might not be that high when it does.

Signed-off-by: Ville Skyttä <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: scop

contrib/completion/git-prompt.sh

@@ -141,7 +141,7 @@ __git_ps1_show_upstream ()
 
 	# parse configuration values
 	local option
-	for option in ${GIT_PS1_SHOWUPSTREAM}; do
+	for option in ${GIT_PS1_SHOWUPSTREAM-}; do
 		case "$option" in
 		git|svn) upstream_type="$option" ;;
 		verbose) verbose=1 ;;