read_early_config(): avoid .git/config hack when unneeded

read_early_config(): avoid .git/config hack when unneeded

So far, we only look whether the startup_info claims to have seen a
git_dir.

However, do_git_config_sequence() (and consequently the
git_config_with_options() call used by read_early_config() asks the
have_git_dir() function whether we have a .git/ directory, which in turn
also looks at git_dir and at the environment variable GIT_DIR. And when
this is the case, the repository config is handled already, so we do not
have to do that again explicitly.

Let's just use the same function, have_git_dir(), to determine whether we
have to handle .git/config explicitly.

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