rebase --keep-empty: always use interactive rebase

rebase --keep-empty: always use interactive rebase

rebase --merge accepts --keep-empty but just ignores it, by using an
implicit interactive rebase the user still gets the rename detection
of a merge based rebase but with with --keep-empty support.

If rebase --keep-empty without --interactive or --merge stops for the
user to resolve merge conflicts then 'git rebase --continue' will
fail. This is because it uses a different code path that does not
create $git_dir/rebase-apply. As rebase --keep-empty was implemented
using cherry-pick it has never supported the am options and now that
interactive rebases support --signoff there is no loss of
functionality by using an implicit interactive rebase.

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