date: document and test "raw-local" mode

The "raw" format shows a Unix epoch timestamp, but with a
timezone tacked on. The timestamp is not _in_ that zone, but
it is extra information about the time (by default, the zone
the author was in).

The documentation claims that "raw-local" does not work. It
does, but the end result is rather subtle. Let's describe it
in better detail, and test to make sure it works (namely,
the epoch time doesn't change, but the zone does).

While we are rewording the documentation in this area, let's
not use the phrase "does not work" for the remaining option,
"--date=relative". It's vague; do we accept it or not? We do
accept it, but it has no effect (which is a reasonable
outcome). We should also refer to the option not as
"--relative" (which is the historical synonym, and does not
take "-local" at all), but as "--date=relative".

Helped-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -725,8 +725,8 @@ include::pretty-options.txt[]
 	`iso-local`), the user's local time zone is used instead.
 +
 `--date=relative` shows dates relative to the current time,
-e.g. ``2 hours ago''. The `-local` option cannot be used with
-`--raw` or `--relative`.
+e.g. ``2 hours ago''. The `-local` option has no effect for
+`--date=relative`.
 +
 `--date=local` is an alias for `--date=default-local`.
 +
@@ -747,6 +747,9 @@ format, often found in email messages.
 `--date=short` shows only the date, but not the time, in `YYYY-MM-DD` format.
 +
 `--date=raw` shows the date in the internal raw Git format `%s %z` format.
+Note that the `-local` option does not affect the seconds-since-epoch
+value (which is always measured in UTC), but does switch the accompanying
+timezone value.
 +
 `--date=format:...` feeds the format `...` to your system `strftime`.
 Use `--date=format:%c` to show the date in your system locale's

t/t0006-date.sh

@@ -47,6 +47,7 @@ check_show short "$TIME" '2016-06-15'
 check_show default "$TIME" 'Wed Jun 15 16:13:20 2016 +0200'
 check_show raw "$TIME" '1466000000 +0200'
 check_show iso-local "$TIME" '2016-06-15 14:13:20 +0000'
+check_show raw-local "$TIME" '1466000000 +0000'
 
 # arbitrary time absurdly far in the future
 FUTURE="5758122296 -0400"