parse-options: simplify positivation handling

parse-options: simplify positivation handling

We accept the positive version of options whose long name starts with
"no-" and are defined without the flag PARSE_OPT_NONEG.  E.g. git clone
has an explicitly defined --no-checkout option and also implicitly
accepts --checkout to override it.

parse_long_opt() handles that by restarting the option matching with the
positive version when it finds that only the current option definition
starts with "no-", but not the user-supplied argument.  This code is
located almost at the end of the matching logic.

Avoid the need for a restart by moving the code up.  We don't have to
check the positive arg against the negative long_name at all -- the
"no-" prefix of the latter makes a match impossible.  Skip it and toggle
OPT_UNSET right away to simplify the control flow.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>