t4209: factor out helper function test_log()
t4209: factor out helper function test_log()

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 <2da278a99b557fc93ddbed7517a413ad793e2e6a@web.de>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git