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".

---
I noticed this messing up my "prove" output. And it always runs first with "prove --state=slow", because it has a whopping 17 seconds of sleeps in it.

 t/t7063-status-untracked-cache.sh | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

index 2b2ffd7..bd4806c 100755
@@ -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

2.4.1.552.g6de66a4