pretty=format: respect date format options

pretty=format: respect date format options

When running a command like:

  git log --pretty=format:%ad --date=short

the date option was ignored. This patch causes it to use whatever
format was specified by --date (or by --relative-date, etc), just
as the non-user formats would do.

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