test-lib-functions: support "test_config -C <dir> ..."

test-lib-functions: support "test_config -C <dir> ..."

If used in a subshell, test_config cannot unset variables at the end of
a test.  This is a problem when testing submodules because we do not
want to "cd" at to top level of a test script in order to run the
command inside the submodule.

Add a "-C" option to test_config (and test_unconfig) so that test_config
can be kept outside subshells and still affect subrepositories.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>