check_repository_format_gently: stop using git_config_early

check_repository_format_gently: stop using git_config_early

There's a chicken-and-egg problem with using the regular
git_config during the repository setup process. We get
around it here by using a special interface that lets us
specify the per-repo config, and avoid calling
git_pathdup().

But this interface doesn't actually make sense. It will look
in the system and per-user config, too; we definitely would
not want to accept a core.repositoryformatversion from
there.

The git_config_from_file interface is a better match, as it
lets us look at a single file.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>