show-branch: don't use LASTARG_DEFAULT with OPTARG

show-branch: don't use LASTARG_DEFAULT with OPTARG

5734365 (show-branch: migrate to parse-options API 2009-05-21)
incorrectly set the --more option's flags to be
PARSE_OPT_LASTARG_DEFAULT and PARSE_OPT_OPTARG. These two flags
shouldn't be used together. An option taking a default should just set
the default value desired and parse options will take care of the rest.

Update the header comment to better convey this information.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Acked-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>