parse-options: normalize arg and long_name before comparison

parse-options: normalize arg and long_name before comparison

Strip "no-" from arg and long_name before comparing them.  This way we
no longer have to repeat the comparison with an offset of 3 for negated
arguments.

Note that we must not modify the "flags" value, which tracks whether arg
is negated, inside the loop.  When registering "--n", "--no" or "--no-"
as abbreviation for any negative option, we used to OR it with OPT_UNSET
and end the loop.  We can simply hard-code OPT_UNSET and leave flags
unchanged instead.

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