approxidate: make "today" wrap to midnight

Although some commands do reject invalid approxidate expressions,
in other cases those are simply evaluated as the current time.
Oftentimes that is a perfectly good compromise to handle silly
requests, but it isn't without rough edges.

Because of the silent acceptance, it is easy to forget that
"today" isn't actually a valid approxidate format.  That is
a bit awkward because while the fallback logic of using the
current time does make some sense, there is no deliberative
decision behind such behavior of "today".  Indeed, whatever
(non-)action "today" currently has, is just an accidental
side effect.

That means "git log --since=today" is currently unlikely to
print anything at all as it tries to list commits dated with
*future* timestamps.  Arguably it would be more useful to
list the commits of the current day---i.e. those made since
midnight.

On the other hand, "git log --until=today" doesn't really
filter commits at all.  Changing the definition of "today"
would make it return the commits made before the current day.
That isn't without problems though---running "git log
--until=today" in the late afternoon could reasonably include
the work done earlier that day (as the command currently
does do).

Still the utility of no-op "--until=today" is debatable and
perhaps outweighed by the pros of having "--since=today" to
mean "--since=midnight".  The thing is that the approxidate
machinery doesn't know about its consumers, so the meaning
of "today" has to be the same for "--since" and "--until".

In fact, "git log --until=" is documented as

	`--until=<date>`::
	`--before=<date>`::
		Show commits older than _<date>_,

so excluding commits made today would actually match the
documentation more closely.

Moreover, a revision parameter "@{today}" is currently outright
rejected.  Making "today" a valid approxidate time format could
make a natural way to specify the state of the ref at the start
of the current day.

Bind "today" to new function `date_today()` as an approxidate
special.  Make it return the last midnight if no specific time
is given; i.e. retain the old behavior of "noon today" and such.

Document the new behavior of "git log --since=today" in
rev-list-options.adoc.

Signed-off-by: Tuomas Ahola <taahol@utu.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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