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>