As we add new options that operate on objects before
replacing them, we'll want to be able to feed raw sha1s
straight into replace_object. Split replace_object into the
object-resolution part and the actual replacement.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>