pretty: add %(describe)
Add a format placeholder for describe output. Implement it by actually
calling git describe, which is simple and guarantees correctness. It's
intended to be used with $Format:...$ in files with the attribute
export-subst and git archive. It can also be used with git log etc.,
even though that's going to be slow due to the fork for each commit.
Suggested-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>