I noticed that a cron-launched "git-clone --quiet" was generating
progress output to standard error -- and thus always spamming me.
The offending output was due to git-clone invoking git-read-tree with
its undocumented -v option.
This change turns off "-v" for --quiet.
Signed-off-by: Jim Meyering <jim@meyering.net> Signed-off-by: Junio C Hamano <junkio@cox.net>