git-stash: use stdout instead of stderr for non error messages

git-stash: use stdout instead of stderr for non error messages

Some scripts and libraries check stderr to detect a failing command,
instead of checking the exit code.  Because the output from git-status
is not primarily for machine consumption, it would not hurt to send
these messages to stdout instead and it will make it easier to drive
the command for such callers.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>