completion: don't leak variable from the prompt into environment

Commit e5b8eebc (completion: fix issue with process substitution not
working on Git for Windows, 2011-10-26) introduced a new variable in
__git_ps1_show_upstream(), but didn't declare it as local to prevent it
from leaking into the environment.

We may want to rewrite it like the following, but that can wait until the
next cycle.

	while read key value
	do
		...
	done <<-EOF
	$(git config -z --get-regexp ...)
	EOF

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -110,7 +110,7 @@ __git_ps1_show_upstream ()
 	local upstream=git legacy="" verbose=""
 
 	# get some config options from git-config
-	output="$(git config -z --get-regexp '^(svn-remote\..*\.url|bash\.showupstream)$' 2>/dev/null | tr '\0\n' '\n ')"
+	local output="$(git config -z --get-regexp '^(svn-remote\..*\.url|bash\.showupstream)$' 2>/dev/null | tr '\0\n' '\n ')"
 	while read key value; do
 		case "$key" in
 		bash.showupstream)