parse-options: disable option abbreviation with PARSE_OPT_KEEP_UNKNOWN

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>