Avoid accessing non-tag refs in git-describe unless --all is requested

Avoid accessing non-tag refs in git-describe unless --all is requested

If we aren't going to use a ref there is no reason for us to open
its object from the object database.  This avoids opening any of
the head commits reachable from refs/heads/ unless they are also
reachable through the commit we have been asked to describe and
we need to walk through it to find a tag.

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