date.c: skip fractional second part of ISO-8601

date.c: skip fractional second part of ISO-8601

git-commit(1) says ISO-8601 is one of our supported date format.

ISO-8601 allows timestamps to have a fractional number of seconds.
We represent time only in terms of whole seconds, so we never bothered
parsing fractional seconds. However, it's better for us to parse and
throw away the fractional part than to refuse to parse the timestamp
at all.

And refusing parsing fractional second part may confuse the parse to
think fractional and timezone as day and month in this example:

	2008-02-14 20:30:45.019-04:00

While doing this, make sure that we only interpret the number after the
second and the dot as fractional when and only when the date is known,
since only ISO-8601 allows the fractional part, and we've taught our
users to interpret "12:34:56.7.days.ago" as a way to specify a time
relative to current time.

Reported-by: Brian M. Carlson <sandals@crustytoothpaste.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>