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>