t7063: hide stderr from setup inside prereq

When t7063 starts, it runs "update-index --untracked-cache"
to see if we support the untracked cache. Its output goes
straight to stderr, even if the test is not run with "-v".
Let's wrap it in a prereq that will hide the output by
default, but show it with "-v".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7063-status-untracked-cache.sh

@@ -8,10 +8,14 @@ avoid_racy() {
 	sleep 1
 }
 
-git update-index --untracked-cache
 # It's fine if git update-index returns an error code other than one,
 # it'll be caught in the first test.
-if test $? -eq 1; then
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+if ! test_have_prereq UNTRACKED_CACHE; then
 	skip_all='This system does not support untracked cache'
 	test_done
 fi