Add `human` format to test-tool

Add `human` format to test-tool

Add the human format support to the test tool so that
GIT_TEST_DATE_NOW can be used to specify the current time.

The get_time() helper function was created and and checks the
GIT_TEST_DATE_NOW environment variable.  If GIT_TEST_DATE_NOW is set,
then that date is used instead of the date returned by by
gettimeofday().

All calls to gettimeofday() were replaced by calls to get_time().

Renamed occurances of TEST_DATE_NOW to GIT_TEST_DATE_NOW since the
variable is now used in the get binary and not just in the test-tool.

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