test-config: setup git directory

test-config: setup git directory

The t1308 test script uses our test-config helper to read
repository-level config, but never actually sets up the
repository. This works because git_config() blindly reads
".git/config" even if we have not configured a repository.

This means that test-config won't work from a subdirectory,
though since it's just a helper for the test scripts, that's
not a big deal.

More important is that the behavior of git_config() is going
to change, and we want to make sure that t1308 continues to
work. We can just use setup_git_directory(), and not the
gentle form; there's no point in being flexible, as it's
just a helper for the tests.

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