stash: don't show internal implementation details

stash: don't show internal implementation details

git stash push uses other git commands internally.  Currently it only
passes the -q flag to those if the -q flag is passed to git stash.  when
using 'git stash push -p -q --no-keep-index', it doesn't even pass the
flag on to the internal reset at all.

It really is enough for the user to know that the stash is created,
without bothering them with the internal details of what's happening.
Always pass the -q flag to the internal git clean and git reset
commands, to avoid unnecessary and potentially confusing output.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>