bisect--helper: use OPT_CMDMODE instead of OPT_BOOL

bisect--helper: use OPT_CMDMODE instead of OPT_BOOL

`--next-all` is meant to be used as a subcommand to support multiple
"operation mode" though the current implementation does not contain any
other subcommand along side with `--next-all` but further commits will
include some more subcommands.

Helped-by: Johannes Schindelin <redacted>
Mentored-by: Lars Schneider <redacted>
Mentored-by: Christian Couder <redacted>
Signed-off-by: Pranit Bauva <redacted>
Signed-off-by: Junio C Hamano <redacted>