pretty: add merge and exclude options to %(describe)

pretty: add merge and exclude options to %(describe)

Allow restricting the tags used by the placeholder %(describe) with the
options match and exclude.  E.g. the following command describes the
current commit using official version tags, without those for release
candidates:

   $ git log -1 --format='%(describe:match=v[0-9]*,exclude=*rc*)'

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>