contrib/subtree: ignore log.date configuration
git-subtree's log format string uses "%ad" and "%cd", which
respect the user's configured log.date value.
This is problematic for git-subtree because it needs to use real
dates so that copied commits come through unchanged.
Add a test and tweak the format strings to use %aD and %cD
so that the default date format is used instead.
Reported-by: Bryan Jacobs <b@q3q.us>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>