Twelve tests in t4209 follow the same simple pattern for description,
git log call and checking. Extract that shared logic into a helper
function named test_log. Test specifications become a lot more
compact, new tests can be added more easily.
Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>