date parsing: be friendlier to our European friends.

date parsing: be friendlier to our European friends.

This does three things, only applies to cases where the user
manually tries to override the author/commit time by environment
variables, with non-ISO, non-2822 format date-string:

 - Refuses to use the interpretation to put the date in the
   future; recent kernel history has a commit made with
   10/03/2006 which is recorded as October 3rd.

 - Adds '.' as the possible year-month-date separator.  We
   learned from our European friends on the #git channel that
   dd.mm.yyyy is the norm there.

 - When the separator is '.', we prefer dd.mm.yyyy over
   mm.dd.yyyy; otherwise mm/dd/yy[yy] takes precedence over
   dd/mm/yy[yy].

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