Quit passing 'now' to date code

Quit passing 'now' to date code

Commit b841d4ff43 (Add `human` format to test-tool, 2019-01-28) added
a get_time() function which allows $GIT_TEST_DATE_NOW in the
environment to override the current time. So we no longer need to
interpret that variable in cmd__date().

Therefore, we can stop passing the "now" parameter down through the
date functions, since nobody uses them. Note that we do need to make
sure all of the previous callers that took a "now" parameter are
correctly using get_time().

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