Emit helpful status for accidental "git stash" save

Emit helpful status for accidental "git stash" save

If the user types "git stash" mistakenly thinking that this will list
their stashes he/she may be surprised to see that it actually saved
a new stash and reset their working tree and index.

In the worst case they might not know how to recover the state. So
help them by telling them exactly what was saved and also how to
restore it immediately.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>