stash list: use new %g formats instead of sed

stash list: use new %g formats instead of sed

With the new formats, we can rewrite 'git stash list' in terms of an
appropriate pretty format, instead of hand-editing with sed.  This has
the advantage that it obeys the normal settings for git-log, notably
the pager.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>