bisect--helper: parse subcommand with OPT_SUBCOMMAND

bisect--helper: parse subcommand with OPT_SUBCOMMAND

As of it is, we're parsing subcommand with OPT_CMDMODE, which will
continue to parse more options even if the command has been found.

When we're running "git bisect run" with a command that expecting
a "--log" or "--no-log" arguments, or one of those "--bisect-..."
arguments, bisect--helper may mistakenly think those options are
bisect--helper's option.

We may fix those problems by passing "--" when calling from
git-bisect.sh, and skip that "--" in bisect--helper. However, it may
interfere with user's "--".

Let's parse subcommand with OPT_SUBCOMMAND since that API was born for
this specific use-case.

Reported-by: Lukáš Doktor <ldoktor@redhat.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>