parse-options: disable option abbreviation with PARSE_OPT_KEEP_UNKNOWN

parse-options can unambiguously find an abbreviation only if it sees all available options. This is usually the case when you use parse_options(). But there are other callers like blame or shortlog which uses parse_options_start() in combination with a custom option parser, like rev-list. parse-options cannot see all options in this case and will get abbrev detection wrong. Disable it. t7800 needs update because --symlink no longer expands to --symlinks and will be passed down to git-diff, which will not recognize it. I still think this is the correct thing to do. But if --symlink has been actually used in the wild, we would just add an option alias for it. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>