If you set this for a given option, and the optoin appears without an
argument on the command line, then the `defval' is used as its argument.

Note that this flag is meaningless in presence of OPTARG or NOARG flags.
(in the current implementation it will be ignored, but don't rely on it).
Signed-off-by: NPierre Habouzit <madcoder@debian.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>