builtin-replace: teach "git replace" to actually replace

Teach the syntax: "git replace <object> <replacement>", so that
"git replace" can now create replace refs. These replace refs
will be used by read_sha1_file to substitute <object> with
<replacement> for most of the commands.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>