read-tree: deprecate syntax without tree-ish args

Currently, read-tree can be run without tree-ish arguments, in which
case it will empty the index. Since this behavior is undocumented and
perhaps a bit too invasive to be the "default" action for read-tree,
deprecate it in favor of a new --empty option that does the same thing.

Signed-off-by: Jan Krüger <jk@jk.gs>
Signed-off-by: Junio C Hamano <gitster@pobox.com>