upload-pack: avoid parsing objects during ref advertisement

upload-pack: avoid parsing objects during ref advertisement

When we advertise a ref, the first thing we do is parse the
pointed-to object. This gives us two things:

  1. a "struct object" we can use to store flags

  2. the type of the object, so we know whether we need to
     dereference it as a tag

Instead, we can just use lookup_unknown_object to get an
object struct, and then fill in just the type field using
sha1_object_info (which, in the case of packed files, can
find the information without actually inflating the object
data).

This can save time if you have a large number of refs, and
the client isn't actually going to request those refs (e.g.,
because most of them are already up-to-date).

The downside is that we are no longer verifying objects that
we advertise by fully parsing them (however, we do still
know we actually have them, because sha1_object_info must
find them to get the type). While we might fail to detect a
corrupt object here, if the client actually fetches the
object, we will parse (and verify) it then.

On a repository with 120K refs, the advertisement portion of
upload-pack goes from ~3.4s to 3.2s (the failure to speed up
more is largely due to the fact that most of these refs are
tags, which need dereferenced to find the tag destination
anyway).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>