parse-options: detect ambiguous self-negation

Git currently does not detect the ambiguity of an option that starts with "no" like --notes and its negated form if given just --n or --no. All Git commands with such options have other negatable options, and we detect the ambiguity with them, so that's currently only a potential problem for scripts that use git rev-parse --parseopt. Let's fix it nevertheless, as there's no need for that confusion. To detect the ambiguity we have to loosen the check in register_abbrev(), as an option is considered an alias of itself. Add non-matching negation flags as a criterion to recognize an option being ambiguous with its negated form. And we need to keep going after finding a non-negated option as an abbreviated candidate and perform the negation checks in the same loop. Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>