pretty: add tag option to %(describe)

pretty: add tag option to %(describe)

The %(describe) placeholder by default, like `git describe`, only
supports annotated tags. However, some people do use lightweight tags
for releases, and would like to describe those anyway. The command line
tool has an option to support this.

Teach the placeholder to support this as well.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>