t1411: add more selector index/date tests

t1411: add more selector index/date tests

We already check that @{now} and "--date" cause the
displayed selector to use the date for both the multiline
and oneline formats. However, we miss several cases:

  1. The --format=%gd selector is not tested at all.

  2. We do not check how the log.date config interacts with the
     "--date" magic (according to f4ea32f, it should not
     impact the output).

Doing so reveals that the combination of both (log.date
combined with the %gd format) does not behave as expected.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>