fix config reading in tests

fix config reading in tests

Previously, we set the GIT_CONFIG environment variable in
our tests so that only that file was read. However, setting
it to a static value is not correct, since we are not
necessarily always in the same directory; instead, we want
the usual git config file lookup to happen.

To do this, we stop setting GIT_CONFIG, which means that we
must now suppress the reading of the system-wide and user
configs.

This exposes an incorrect test in t1500, which is also
fixed (the incorrect test worked because we were failing to
read the core.bare value from the config file, since the
GIT_CONFIG variable was pointing us to the wrong file).

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