git-name-rev: add a --(no-)undefined option.

git-name-rev: add a --(no-)undefined option.

Rework get_rev_name to return NULL rather than "undefined" when a
reference is undefined. If --undefined is passed (default), git-name-rev
prints "undefined" for the name, else it die()s.

Make git-describe use --no-undefined when calling git-name-rev so
that --contains behavior matches the standard git-describe one.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>