bisect--helper: identify as bisect when report error

bisect--helper: identify as bisect when report error

In a later change, we will convert the bisect--helper to be builtin
bisect. Let's start by self-identifying it's the real bisect when reporting
error.

This change is safe since 'git bisect--helper' is an implementation
detail, users aren't expected to call 'git bisect--helper'.

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