parse-options: Don't call parse_options_check() so much

parse-options: Don't call parse_options_check() so much

parse_options_check() is being called for each invocation of
parse_options_step which can be quite a bit for some commands. The
commit introducing this function cb9d398 (parse-options: add
parse_options_check to validate option specs., 2009-06-09) had the
correct motivation and explicitly states that parse_options_check()
should be called from parse_options_start(). However, the implementation
differs from the motivation. Fix it.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>