test-lib.sh: unset XDG_CONFIG_HOME

Now that git respects XDG_CONFIG_HOME for some lookups, we
must be sure to cleanse the test environment. Otherwise, the
user's XDG_CONFIG_HOME could influence the test results.

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

t/test-lib.sh

@@ -61,6 +61,7 @@ unset VISUAL EMAIL LANGUAGE COLUMNS $(perl -e '
 	my @vars = grep(/^GIT_/ && !/^GIT_($ok)/o, @env);
 	print join("\n", @vars);
 ')
+unset XDG_CONFIG_HOME
 GIT_AUTHOR_EMAIL=author@example.com
 GIT_AUTHOR_NAME='A U Thor'
 GIT_COMMITTER_EMAIL=committer@example.com