setup.c: don't setup in discover_git_directory()

setup.c: don't setup in discover_git_directory()

discover_git_directory() started modifying the_repository in ebaf3bcf1ae
(repository: move global r_f_p_c to repo struct, 2021-06-17), when, in
the repository setup process, we started copying members from the
"struct repository_format" we're inspecting to the appropriate "struct
repository". However, discover_git_directory() isn't actually used in
the setup process (its only caller in the Git binary is
read_early_config()), so it shouldn't be doing this setup at all!

As explained by 16ac8b8db6 (setup: introduce the
discover_git_directory() function, 2017-03-13) and the comment on its
declaration, discover_git_directory() is intended to be an entrypoint
into setup.c machinery that allows the Git directory to be discovered
without side effects, e.g. so that read_early_config() can read
".git/config" before the_repository has been set up.

Fortunately, we didn't start to rely on this unintended behavior between
then and now, so we let's just remove it. It isn't harming anyone, but
it's confusing.

Signed-off-by: Glen Choo <chooglen@google.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>