bisect: remove Cogito-related code

bisect: remove Cogito-related code

Once upon a time, there was this idea that Git would not actually be a
single coherent program, but rather a set of low-level programs that
users cobble together via shell scripts, or develop high-level user
interfaces for Git, or both.

Cogito was such a high-level user interface, incidentally implemented
via shell scripts that cobble together Git calls.

It did turn out relatively quickly that Git would much rather provide a
useful high-level user interface itself.

As of April 19th, 2007, Cogito was therefore discontinued (see
https://lore.kernel.org/git/20070419124648.GL4489@pasky.or.cz/).

Nevertheless, for almost 15 years after that announcement, Git carried
special code in `git bisect` to accommodate Cogito.

Since it is beyond doubt that there are no more Cogito users, let's
remove the last remnant of Cogito-accommodating code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>