Use RFC2822 dates from "git fmt-patch".

Still Work-in-progress git fmt-patch (should it be known as
format-patch-ng?) is matched with the fix made by Huw Davies
in 262a6ef76a1dde97ab50d79fa5cd6d3f9f125765 commit to use
RFC2822 date format.

Signed-off-by: Junio C Hamano <junkio@cox.net>