config: provide a version of git_config with more options

config: provide a version of git_config with more options

Callers may want to provide a specific version of a file in which to look
for config. Right now this can be done by setting the magic global
config_exclusive_filename variable.  By providing a version of git_config
that takes a filename, we can take a step towards making this magic global
go away.

Furthermore, by providing a more "advanced" interface, we now have a a
natural place to add new options for callers like git-config, which care
about tweaking the specifics of config lookup, without disturbing the
large number of "simple" users (i.e., every other part of git).

The astute reader of this patch may notice that the logic for handling
config_exclusive_filename was taken out of git_config_early, but added
into git_config. This means that git_config_early will no longer respect
config_exclusive_filename.  That's OK, because the only other caller of
git_config_early is check_repository_format_gently, but the only function
which sets config_exclusive_filename is cmd_config, which does not call
check_repository_format_gently (and if it did, it would have been a bug,
anyway, as we would be checking the repository format in the wrong file).

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