rebase -i --keep-empty: don't prune empty commits

rebase -i --keep-empty: don't prune empty commits

If there are empty commits on the left hand side of $upstream...HEAD
then the empty commits on the right hand side that we want to keep are
pruned by --cherry-pick. Fix this by using --cherry-mark instead of
--cherry-pick and keeping the commits that are empty or are not marked
as cherry-picks.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>