git-describe: use tags found in packed-refs correctly

git-describe: use tags found in packed-refs correctly

When your refs are packed, "git-describe" can find the tag that is the
best match without ever parsing the tag itself.  But lookup_tag() in
display_name() says "I've never seen it", creates an empty shell, and
returns it.  We need to make sure that we actually have parsed the tag
data into it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>