Add `human` date format tests.

Add `human` date format tests.

When using `human` several fields are suppressed depending on the time
difference between the reference date and the local computer date. In
cases where the difference is less than a year, the year field is
supppressed. If the time is less than a day; the month and year is
suppressed.

Use TEST_DATE_NOW environment variable when using the test-tool to
hold the expected output strings constant.

Signed-off-by: Stephen P. Smith <ischis2@cox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>