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].