Teach git-describe to use peeled ref information when scanning tags

Teach git-describe to use peeled ref information when scanning tags

By using the peeled ref information inside of the packed-refs file we
can avoid opening tag objects to obtain the commits they reference.
This speeds up git-describe when there are a large number of tags
in the repository as we have less objects to parse before we can
start commit matching.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>