date/time: do not get confused by fractional seconds

The date/time parsing code was confused if the input time HH:MM:SS is
followed by fractional seconds.  Since we do not record anything finer
grained than seconds, we could just drop fractional part, but there is a
twist.

We have taught people that not just spaces but dot can be used as word
separators when spelling things like:

    $ git log --since 2.days
    $ git show @{12:34:56.7.days.ago}

and we shouldn't mistake "7" in the latter example as a fraction and
discard it.

The rules are:

 - valid days of month/mday are always single or double digits.

 - valid years are either two or four digits

   No, we don't support the year 600 _anyway_, since our encoding is based
   on the UNIX epoch, and the day we worry about the year 10,000 is far
   away and we can raise the limit to five digits when we get closer.

 - Other numbers (eg "600 days ago") can have any number of digits, but
   they cannot start with a zero. Again, the only exception is for
   two-digit numbers, since that is fairly common for dates ("Dec 01" is
   not unheard of)

So that means that any milli- or micro-second would be thrown out just
because the number of digits shows that it cannot be an interesting date.

A milli- or micro-second can obviously be a perfectly fine number
according to the rules above, as long as it doesn't start with a '0'. So
if we have

	12:34:56.123

then that '123' gets parsed as a number, and we remember it. But because
it's bigger than 31, we'll never use it as such _unless_ there is
something after it to trigger that use.

So you can say "12:34:56.123.days.ago", and because of the "days", that
123 will actually be meaninful now.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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