parse-options.c: use exhaustive "case" arms for "enum parse_opt_result"

parse-options.c: use exhaustive "case" arms for "enum parse_opt_result"

Change the "default" case in parse_options() that handles the return
value of parse_options_step() to simply have a "case" arm for
PARSE_OPT_UNKNOWN, instead of leaving it to a comment. This means the
compiler can warn us about any missing case arms.

This adjusts code added in ff43ec3e2d2 (parse-opt: create
parse_options_step., 2008-06-23), given its age it may pre-date the
existence (or widespread use) of this coding style, which we've since
adopted more widely.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>