contrib/subtree: ignore log.date configuration

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 <redacted>
Signed-off-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>