Add log.date config variable

log.date config variable sets the default date-time mode for the log
command. Setting log.date value is similar to using git log's --date
option.

Signed-off-by: Heikki Orsila <heikki.orsila@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -817,6 +817,12 @@ instaweb.port::
 	The port number to bind the gitweb httpd to. See
 	linkgit:git-instaweb[1].
 
+log.date::
+	Set default date-time mode for the log command. Setting log.date
+	value is similar to using git log's --date option. The value is one of
+	following alternatives: {relative,local,default,iso,rfc,short}.
+	See linkgit:git-log[1].
+
 log.showroot::
 	If true, the initial commit will be shown as a big creation event.
 	This is equivalent to a diff against an empty tree.

Documentation/rev-list-options.txt

@@ -16,7 +16,8 @@ include::pretty-options.txt[]
 --date={relative,local,default,iso,rfc}::
 
 	Only takes effect for dates shown in human-readable format, such
-	as when using "--pretty".
+	as when using "--pretty". `log.date` config variable sets a default
+	value for log command's --date option.
 +
 `--date=relative` shows dates relative to the current time,
 e.g. "2 hours ago".