The test-date program goes back to the early days of git,
where it was presumably used to do manual sanity checks on
changes to the date code. However, it is not actually used
by the test suite to do any sort of automatic of systematic
tests.

This patch refactors the interface to the program to try to
make it more suitable for use by the test suite. There
should be no fallouts to changing the interface since it is
not actually installed and is not internally called by any
other programs.

The changes are:

  - add a "mode" parameter so the caller can specify which
    operation to test

  - add a mode to test relative date output from show_date

  - allow faking a fixed time via the TEST_DATE_NOW
    environment variable, which allows consistent automated
    testing

  - drop the use of ctime for showing dates in favor of our
    internal iso8601 printing routines. The ctime output is
    somewhat redundant (because of the day-of-week) which
    makes writing test cases more annoying.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>