t9400: Use the repository config and nothing else.

t9400: Use the repository config and nothing else.

git-cvsserver has a bug in its configuration file output parser
that makes it choke if the configuration has these:

        [diff]
                color = auto
        [diff.color]
                whitespace = blue reverse

This needs to be fixed, but thanks to that bug, a separate bug
in t9400 test script was discovered.  The test discarded
GIT_CONFIG instead of pointing at the proper one to be used in
the exoprted repository.  This allowed user's .gitconfig and (if
exists) systemwide /etc/gitconfig to affect the outcome of the
test, which is a big no-no.

The patch fixes the problem in the test.  Fixing the
git-cvsserver's configuration parser is left as an exercise to
motivated volunteers ;-)

Signed-off-by: Junio C Hamano <junkio@cox.net>