git-am: Add --ignore-date option

git-am: Add --ignore-date option

This new option tells 'git-am' to ignore the date header field
recorded in the format-patch output. The commits will have the
timestamp when they are created instead.

You can work a lot in one day to accumulate many changes, but
apply and push to the public repository only some of them at
the end of the first day. Then next day you can spend all your
working hours reading comics or chatting with your coworkers,
and apply your remaining patches from the previous day using
this option to pretend that you have been working at the end
of the day.

Signed-off-by: しらいしななこ <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>