stash: implement '--staged' option for 'push' and 'save'

stash: implement '--staged' option for 'push' and 'save'

Stash only the changes that are staged.

This mode allows to easily stash-out for later reuse some changes
unrelated to the current work in progress.

Unlike 'stash push --patch', --staged supports use of any tool to
select the changes to stash-out, including, but not limited to 'git
add --interactive'.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>