bisect--helper: emit usage for "git bisect"

In subsequent commits we'll be removing "git-bisect.sh" in favor of
promoting "bisect--helper" to a "bisect" built-in.

In doing that we'll first need to have it support "git bisect--helper
<cmd>" rather than "git bisect--helper --<cmd>", and then finally have
its "-h" output claim to be "bisect" rather than "bisect--helper".

Instead of suffering that churn let's start claiming to be "git
bisect" now. In just a few commits this will be true, and in the
meantime emitting the "wrong" usage information from the helper is a
small price to pay to avoid the churn.

Let's also declare "BUILTIN_*" macros, when we eventually migrate the
sub-commands themselves to parse_options() we'll be able to re-use the
strings. See 0afd556b2e1 (worktree: define subcommand -h in terms of
command -h, 2022-10-13) for a recent example.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

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