test-lib.sh: unset XDG_CACHE_HOME

git respects XDG_CACHE_HOME for the credential cache. So, we should
unset XDG_CACHE_HOME for the test environment, lest a user's custom one
cause failure in the test.

For example, t/t0301-credential-cache.sh expects a default directory
to be used if it hasn't explicitly set XDG_CACHE_HOME.

Signed-off-by: Genki Sky <sky@genki.is>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -104,6 +104,7 @@ unset VISUAL EMAIL LANGUAGE COLUMNS $("$PERL_PATH" -e '
 	my @vars = grep(/^GIT_/ && !/^GIT_($ok)/o, @env);
 	print join("\n", @vars);
 ')
+unset XDG_CACHE_HOME
 unset XDG_CONFIG_HOME
 unset GITPERLLIB
 GIT_AUTHOR_EMAIL=author@example.com